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Lab rat Siblings (Kinkylicious & sinfulrook)

T

Tentacle_Lover

Supernova
Joined
Mar 9, 2009
"Ugh, can you believe him?" Sandra rolled her eyes as she opened up the door to the house, walking inside and folding her arms. "Ugh, dad invites us for a trip down to his lab, tells us it's going to be the trip of a lifetime, and what happens? He talks us into being his lab rats. AGAIN! I swear, there's no way I'm ever eating or drinking a thing that dad gives me again..." The girl rolled her eyes, slipping off her shoes and glancing back at her twin brother. "You gotta agree right? I mean he didn't even tell me what that stuff he gave us even was. Ugh...." She groaned again, then set down the box that their father had given her, supposedly containing more of the drug he had given them earlier, insisting that they take some more later incase it would take multiple doses, though Sandra didn't really intend on taking another drop.

"Did he even tell you what this stupid 'experiment' was all supposed to be about? Or is he just trying to freak me out again and it was just some weird soda like last time? Ugh...." She walked over to a mirror, starting to examine herself. "I swear if my hair starts changing colour again I'm going to kill him...." The 16 year old girl muttered as she examined her hair and then just turned to head through to the main room.
 
Leo followed his twin sister Sandra into their home, listening to her whine and groan about what their dad did to them today, again. "Yeah, I know what you mean, pisses me off when we know nothing about what he does, and he just invites us to be his test subjects..." He said as he lazily kicked his shoes off and went towards the living room.

Leo was just another 16-year-old teenager, having a normal life mostly, except that his mother was never home, always away on business, and his dad was always at the lab, only home for the holidays. Most of the time, Leo and his twin sister had to do everything themselves, other than pay the bills. "I tried asking one of his colleagues, they didn't even hint at what it was." He said and walked into the living room, sitting down on the couch with a loud thud and turning on the TV. He flipped through channels until a comedy show came up. When he found the channel, he put the remote down and then began to stretch his semi-muscular body.

Afterwards, he ran his hand through his short brown hair a few times, his sky blue eyes looking over at his sister. "I bet it was just a placebo like the last two times..." He told her and waited for her to sit down.
 
"Yeah probably...." Sandra sighed and flopped down onto the couch beside him, still muttering under her breath. "Then again he's kinda missing the point. A Placebo only works if you actually tell someone what it's supposed to do...." She rolled her eyes, then started to look around for the TV remote. "And to make it even worse he doesn't even pay us like I bet he does any other subject who walks in off the street. Just because we're his kids he thinks we're free lab rats for him. Ugh. The next time he's home I'm really gonna make him pay that ass...." She sighed, grabbing the remote and lifting it up, just starting to flick through the channels. "Let's see.....bet there's nothing good on either....no...crap...crap...crap...boring....seen it....boring....stupid...."

After a few minutes she just dropped the remote between them. "Here, you pick something to watch. Geez.....500 channels and there's not a damn thing on. I'm gonna get some snacks" The girl stood up, immediately turning to go, her ass right in Leo's face for a second before she started walking. "And don't you dare try putting on some porn or something while I'm gone Leo!"
 
Leo was watching a comedy that he's seen before, but he loved it, and would watch it anytime. When she grabbed the remote and she began to flip through channels, he got a little miffed at her, but then sighed and watched her flip through all the channels. "You watch too much TV, Sandra, I haven't seen half that stuff..." He told her as she put the remote down. He grabbed the remote and watched her get up. Leo noticed her ass right in front of him, and he wanted to smack it to tease her, but her didn't, although he saw something different about it... It was a little bit bigger and rounder...

Sandra probably didn't notice it and thought that she gained weight, but Leo noticed it, it wasn't weight gain, it was something else. He shrugged to himself when she was gone and then flipped back to the comedy he was watching before she changed it. "Hey! Get me something too! Please!" He shouted to her from where he was sitting, hoping she'd be nice to him and bring something for him to snack on as well.
 
"What am I your slave?" Sandra rolled her eyes as she shouted through from the kitchen, but grabbed a can of soda and a bag of chips for her brother as well as herself, then turned to head back through to the living room, wiggling her hips a little. "Do you feel a little weird at all? Since we got home....I dunno....I just feel kinda odd" She shook her head, then gently tossed Leo's soda over to him with a chuckle. It would be an easy catch, but it would ensure that the soda would be well shaken and couldn't be opened quite away. "Next time you can go get your own soda" She chuckled and flopped down beside him, opening up her own soda and taking a long gulp. A few drops of it escaped her lips however, running down her chin and neck, then down into her tanktop, beads of cold fluid running between her small but perky breasts, vanishing down into her clothing.

"Mmmm....that's better.....I feel pretty warm right now too...." She tugged at her top a little, fanning it slightly to get some cool air down it, showing off a little more cleavage along with her bra. "A cold drink is just what I need....ahh.....geez I bet dad put some kind of stupid hot sauce in that stuff or something and it's making me really warm...."
 
Leo snickered a little bit at the slave comment, and thought about what it would be like if she really was his maid. He would be able to be so lazy if he didn't have work, but then again, he wouldn't do that to the one person he loves more than anyone. When she threw the soda at him, he caught it with one hand and sighed, knowing it would be all shaken up. When she plopped down next to him, he put the top of the soda can over his mouth and opened it with his pearly white teeth, the soda spraying into his mouth rather than all over him. His eyes looked over at her and saw the single drop of soda running down her neck and into her shirt. When he looked down at her chest, he watched her open up her shirt and fan it a bit, noticing her somewhat larger cleavage now.

"I always feel weird when I come home from dad's work, but you know... I do feel a bit odd compared to all those previous visits..." He said to her and set his soda aside, feeling a little hot as well. What's going on? It was cool just a minute ago in here... He thought to himself and slowly took off his shirt, showing off his somewhat muscular body. "It's not just me then that was getting hot..." He said and soon felt an erection starting to rise, showing a bulge in his pants already. "If it was hot sauce, I would've broken out in sweat instantly... This is different." He told her and then looked at her again, noticing that her bust was definetely larger than it normally was.
 
"Hm?" Sandra glanced over at Leo as he removed his shirt, raising an eyebrow. "Have you been working out? You look way more muscular than you did a few weeks ago....but I haven't seen you going to the gym or anything..." Sandra blinked, then tutted as she reached down to absent mindedly tug at her bra a little. "Ugh....man....don't tell me this thing is too small....I only just got a few this size the other week...." She muttered and took another drink of her soda, not quite noticing Leo's slowly growing erection bulging out in his pants.

Moments later though, Sandra seemed to freeze up, gasping and suddenly dropping her half-full can, the contents spilling out over her shirt, rendering it semi-transparent. The cold fluid immediately caused her nipples to start hardening, pushing up through her bra and shirt, but they were much bigger than usual, much more noticeable. "HNGH!" Sandra's eyes went wide, her back arching slightly, before suddenly her chest started to visibly swell before Leo's eyes. Slowly her breasts were growing outwards, filling out quickly. Within a few moments, the sound of straining fabric could be heard, before suddenly a loud snapping noise, Sandra's bra giving way as its contents continued to expand, pushing her tank top out and causing it to lift up and reveal more of her smooth belly. By the time the growth spurt stopped, Sandra had suddenly reached an E-cup, her tank top seemingly at the limits of what it could contain.
 
"No, not really." Leo said, not realizing that he's gotten a little bit more muscular. When she whined about her bra, he tried not to look, but when she dropped her half-full can onto her chest, he set his can aside and sat up, looking at her and wondering if she was alright. Leo's eyes then fixated on her growing bust, watching it in amazement as she heaved her chest forward and arched her back. "Sandra!" He said loudly, realizing she couldn't do anything about her bust. He just watched it grow, and when it stopped, he was amazed even more than while she was growing.

"Sandra, are you okay?" He asked her and put his hand on her shoulder, waiting for her answer. Just before she could answer, he let go and then curled up his upper body into his legs, his stomach pressing against his thighs, and his arms wrapped around his stomach. "Nnnn." He groaned in a little bit of discomfort, but it wasn't his stomach, it was his erection. It blew up instantly to a rock solid state and began to grow even larger, the bulge in his pants expanding. After a few seconds, he sat back up again and sighed in relief. He then looked down and noticed his new gigantic bulge, his cock most likely now about 10 inches long, if not bigger.
 
Sandra was panting heavily as she recovered from the strange growth spurt, then slowly glanced over at Leo as he doubled over, gasping as she watched him contort in pain, then slowly sit up and sigh as if relieved, though her eyes were now fixated on the area between his legs. "L-Leo....? Wh-what....what the hell is that thing....?" First of all, why was her twin brother getting aroused infront of her? Second of all, how fucking big was it?! That was like a tent in his pants! "Wh-what's happening to us.....?" Sandra fidgeted slightly, a sudden heat beginning to develop between her legs, juices starting to pour out of her and practically soak the girls thighs. "Nnngh....L-Leo I....I feel really hot...."
 
Leo looked over at her and then down between her legs. Her pants were thoroughly soaked now, and he now knew why. "We had a delayed reaction.... To the drug..." He said to her and then unbuckled his pants, letting his newly sized cock come out. It was throbbing hard from the power of the erection, but it didn't hurt. "It was a hormone drug, this is just the side effect... The growth that is..." He said and just sat there, staring at his own erection. It felt so weird to have something so big inside his pants, he needed to keep it out so it wouldn't hurt. He had no shame in showing it to his sister right now, he felt really weird, he just wanted to jump on her, but he wanted to see what she would do about herself first.
 
Sandra couldn't help but stare at her brothers cock as it sprang free, instinct causing her to subconsciously lick her lips without even realising it. "The....drug? I.....really am gonna kill dad...." She muttered half-heartedly, her fingers starting to twitch. Oh god....why did she want to do this? Leo was her brother! Her twin brother! He was practically the male version of her! But then....maybe that made it ok...? After all she masturbated so....was this really so different? In her distorted mind, that was justification enough for the moment. Before she could even stop herself, Sandra had leaned over, grasping at Leo's cock with both hands. "It....it's so hard....so hot...."
 
Leo watched his sister lean over, and when she grabbed his thick cock with her hands, it twitched, hard and quick. "Your hands feel so nice on it." He said as he put his hands on her wrists and began to move her hands up and down on his shaft. "Ahh, feels so good..." He said with a slight groan of pleasure escaping his throat. He then pulled his hands away from her wrists and put one arm around her, both of his hands going to her breasts and squeezing them gently, feeling just how big and real they really were. "So warm..... and so.... soft...." He muttered while he played with her breasts, her hands still wrapped around his cock. He didn't know what was going on, but he was liking it a lot. His conscience has completely disappeared, his reason not even coming out to tell him that what he was doing was really wrong.
 
Sandra should have known this was wrong, she should have known to stop before this went any further, but for some reason it didn't feel wrong....every argument her mind tried to make was just as quickly justified as being perfectly fine.....no...no this wasn't wrong, nothing was wrong with her slowly working her hands up and down the shaft of her twin brothers cock. "Ahh....it's so hot...so hard..." Sandra leaned a little closer, then gasped as Leo squeezed at her breasts, moaning loudly, they were so much more sensitive than normal! "Nnn...." Slowly, Sandra moved around, now getting onto the floor on her knees infront of Leo. Her hands left his cock just long enough to grasp her breasts, then wrap them around his shaft, beginning to use her newly enhanced assets to slowly wank her sibling.
 
Leo squeezed and massaged her breasts while she stroked his length, but when she got on her knees, he spread his legs for her, completely from instinct. He watched her go down and when she grabbed her new large breasts, he watched as they wrapped around his new cock and began to move up and down, squeezing his cock hard. "Ahhh..." He let out a small sigh of pleasure as she jerked him off with her orbs, watching her the entire time. "Sandra, please... Don't stop... Go faster..." He asked her as best as he could, the pleasure level beyond that of just masturbating on his own. It felt so good that he was going to cum at any moment now, feeling like it was going to be a lot.
 
Sandra had no intentions of stopping. Any instinct or feeling of wrong-doing that might stop her was long since repressed. She just started to move her breasts faster as Leo wanted, moaning as it felt good for her too, the sensation of his hot, hard shaft between her breasts...god why hadn't she done this before? She squeezed a little tighter, bringing extra pleasure to both him and her, her breasts now sliding right from the base of Leo's cock, all the way up to completely smother the head, coating her breasts in his pre-cum, before sliding back down again. "Ahh...it's so hot....so hard...."
 
Leo couldn't help but thrust his hips a bit along with her rubbing, wanting her to rub more and harder. When she was finally rubbing the whole thing, he stopped moving his hips, but instead let out a moan and watched her keep going. There was enough precum to lubricate her cleavage enough for her to go even faster around his cock. "Sandra, I'm really close, I don't think I can hold it in any longer." He warned her, his cum slowly building up more and more, getting that much closer to his limit. Her new, big tits were so soft and warm, there was no way he couldn't cum from that.
 
Sandra either didn't hear what he had said, or his words spurred her on even more. Perhaps it was the arousal, or perhaps it was just the lubrication of Leo's pre-cum, but Sandra was starting to work his cock faster and faster between her breasts, moaning loudly herself, drool occasionally dripping from her lips onto the engorged head of her twin brothers cock. "Nnngh....it's so hot...." Sandra was panting heavily herself, the burning arousal between her own legs only kept at bay by the pleasure she was giving to her chest by wanking him, occasionally pinching at her own nipples and letting out short, sharp hisses of further pleasure.
 
Leo moaned loudly and thrusted his hips forward, further past her breasts. He came, and his cum shot directly at her face, the first spurt of cum strong, and then slowly winding down in a few more shots. His cum covered her face and breasts completely, as well as the exposed part of his cock, dripping down slowly to her hands. After a short while, he sighed and relaxed in his seat, looking down at her with a surprised look on his face. "Sandra... It felt so amazing, and there's so much cum... All over you..." He said to her with a small smile, liking this drug that they took.
 
Sandra gasped when Leo suddenly came, letting out a groan and opening her mouth as his cum sprayed all across her face, plenty landing in her mouth as she savoured the taste, rolling it around on her tongue as he continued cumming, covering her face and the top of her now voluptuous breasts. "Oooh....Leo....so much...." She swallowed his cum, the taste almost like a drug on her tongue, god it was amazing. She couldn't help licking her lips in order to get more. "I.....please Leo..." The girl fidgeted as she continued kneeling infront of him. "I....I need to feel good too.....please...."
 
Leo watched his twin sister roll the cum around in her mouth and then swallow it. He found it to be rather hot how she looked up at him and begged him like that. "Don't worry sis, just lay down on your back here." He said and patted the sofa next to him, waiting for her to oblige. He was going to pay her back for that service, and he was going to make her feel really good, using his new cock that he just got from those drugs. He didn't know whether he should dump his second load inside her, or all over her, but he'll find out soon enough.
 
Sandra didn't need to be told twice, immediately getting up and laying down on the sofa beside him, her breasts now perky enough to still be sticking straight up into the air despite her new position, cum still dripping from the upper half of her body. The girl was still wearing her panties and her skirt, though both had been soaked by her flood of juices since the effects of the drug had taken hold. "P-please Leo....h-hurry....I'm gonna go insane...." The girl was fidgeting, her hands going to grasp at her breasts without her even meaning to, gasping at the pleasure she was giving herself, tweaking at her engorged nipples.
 
Leo watched her lay down and then pulled off her skirt and panties at the same time, tossing them to the floor with the rest of their clothes. He then got on top of her, watching her play with her new large bust, smiling at her and pushing his cock inside her. "S-Sandra, it's so tight!" He told her with a moan as he pushed in slowly, going deeper and deeper. Once his cock was all the way inside her down to the hilt, he began pulling back, then thrusting back inside her faster. He began fucking his twin sister at a medium pace, his hips smacking against hers, making a smack sound echo through the room. He loved it, and he knew she loved it too, but he couldn't say anything, his moans taking over his speech completely.
 
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