I have read about the FAQ, but I would like to ask you something different:
I like that mother and sister offer some resistance, not because they don't want to or want to. I neither want nor expect there to be rape, no. I mean, "out of decency" avoid doing dirty things with your son / brother, even if you want to (Well, no person is made of stone)
So ... As the chapters progress, that resistance will be reduced or there will be the possibility that it will be maintained. Something like a "limit" line that should not be crossed and is constantly crossed. For example: You are my son, I should not drink your milk. He insists and she gives in, because she wanted him to. You are my brother, we can only have anal sex, not vaginal. He seduces her and they end up doing it.
And so with all the perversities that you can think of. What I mean, to make it clear, is that all sexual relationships will have some conflict or if in the long run it will just be putting her in and taking her out.