yeah, BlackSnake improved it very much. The original plot wouldn't work.
It's more like an ultimatum: Either he has sex with the sister or with the mother .... and since the sister wants sex with him, and the mother doesn't .....
The idea that the mother also commited incest many years ago is really good.
Or to make it even better: it wasn't many years ago - it was just a few weeks or months earlier.
There is a big family reunion like Thanksgiving or something like that. The whole family (grandparents, the mother and her kids and of course the brother of the mother, "Uncle Herman") is at the house of the grandparents. At one time, the son sees how his mother and her brother leave the room - and they are in a strange mood. Quietly, he follows them upstairs. They enter one room and lock the door. When he peeps through the keyhole, he sees how they get undressed and how they have sex. Then, he hears his mother "Oh Baby, I missed it so much ...." - so this wasn't the first time.
Later, the son tells his sister about it. Finally, the son asks his sister what she thinks about incest - and after some talking, they have sex.
From that day on, they do it many times, until the mother catches them.
She gets mad and tells them how sick this is - but then, the boy tells her, that he saw how she had sex with her own brother ....
Then he blackmails her: He wants to fuck her, too, otherwise, he would tell her husband (his father) about everything ....
"We have done so much, with so little, for so long, that now we can do almost anything with nothing." - Dave Marcis