(so I have an idea... if you don't mind drawing this out a bit,
A deep moan rose from Sarah's lips, muffled by her sisters body. Her cheeks felt redhot with the woman's thighs pressing up against them snugly. As she tentavily started lapping against Alexis sex, her eyes wide and watery from over stimulation. Lust and need rested deep in the pits of those emerald greed eyes as her body yearned for attention. She let her mind slip away as she continued to lick and please her sisters slit, completely giving herself up to her sisters whims, egged on by the constant teasing from the older girls fingertips.
As the fire raged up inside of her, lust and demand controlling her body, she felt her climax, so long held from her finally start to come to a head, as her sister continued to hold up her end of the bargain, fingers working her over pleasantly. Her mind seemed to drift up and away from her body as she tried to pretend this was all a dream, that she hadn't laid here tormented for hours on end, with little food and sleep and no release.
She felt her heartbeat thunder in her chest, eyes dilating, and fluttering up to the ceiling as another low moan squealed out of the gag her sisters body formed, she heard the loud long rhythm of the doorbell urgently ringing from down the hall, causing her mind to come crashing back down to reality.
"mmphp" she gurgled urgently under her sisters body, eyes suddenly wide with hope that someone had come at least and would see what was going on. A new fiery urge, the need to be free rose up inside of her temporarily overriding her body's desires and desperate needs, she knew if this did int work she'd be punished, but it didn't matter, she had to take the chance.. Had to. "Mmmmph! Hlp!" she tried to scream out, desperate to pull her head away from her sisters sex and get out a call for help.