Taking on a non-con perspective, the world is a very fucked up place and a realistic situation for any incest pairing would be under gunpoint.
A kidnapping/hostage situation can make people do things they never imagined they ever could. A sadistic intruder threatens to blow the daughter’s brains out if the father refuses to have sex with her. Neither of them are willing, but they do not wish to die either.
What father/brother/uncle could sign their own relative’s death warrant? This can be switched for Mother/son, or just about any familial pairing really, and it may seem easy to say you wouldn’t but until you’ve faced with a loved one staring down the barrel of a gun, you truly do not understand what you’d be willing to do if it ensures the room isn’t painted red.
Of course this only works for those willing to write darker themes, but there is always the aftermath, the survival, having to face what they did under duress that can twist things into something more consensual. What starts as harmless comfort from a shared trauma can become something more. Without therapy, your mind can twist in all sorts of ways. Sure, it doesn’t happen to everyone, but who is to say? Every mind is different and perhaps a small piece of the characters snapped in just the right way afterward. Perhaps as taboo, and horrible their experience had been, it had felt good. Wracked with guilt and shame, they resist their intrusive thoughts, the poison, but eventually?