On the contrary, I'm 90% sure it was the uncle who molested the mom when they were kids, not the other way around. Particularly because she, not him, was the one crying at the end.
Why, just because he's a man she's a woman?
What keeps Cindy from coming forward to police about Keith/Molly incest is that her past deamons would come to light: that she would be shown as the one who instigated the incest between her and her brother years ago (aside from the fact about 30 people at the party would testify Ryder raped Molly having seen her run away with blood). We are never told how old Cindy and Keith are, but based on their children's age, it's plausible Cindy is the older sister.
When logic and science aren't on your side, you always lose.