Mother offers daughter lots of money to have sex with own father and to get knocked up by him. One idea could be that the mom is afraid that the daughter may fall into a bad crowd and she has tried different things and paying her to have sex and get impregnated by her father is her last attempt to save her daughter. I do like the other ideas that have been posted in this thread.
Hard to picture that working, given human dynamics. A better (if overused) approach would be having a dying mother 'arrange' for the daughter to seduce the father to take her role after she passes on (i.e. the guy is so horny and incompetent he couldn't possibly survive on his own without someone to corral his overactive libido).
How about a story about a wife who loves, but can't stand to live with her husband, convincing her daughter to become her husband's lover and caretaker. Then once their incestuous relationship is started, the wife divorces him.
Then once their incestuous relationship is started, the wife divorces him.
And more likely, uses evidence of the incestuous relationship to convict the father of sex crimes which will follow him the rest of his life: Divorce can get messy! Puts a whole different spin on the story, though it paints the mother as a bitch sacrificing her daughter just to exact revenge.
Yeah. You could also have Mom jumping into bed with them after the divorce to prove it wasn't personal.
Hard to picture that working, given human dynamics
Here is a situation where it could be plausible.
Mom & dad had their first daughter early, barely out of high school. Mom wants more kids, but can't for medical reasons and isn't keen on adoption.
Of course that situation could be expanded to Mom babysits, daughter goes out to find a boyfriend, marries him, he moves in, and ends with a group sex scene with all saying best sex every and swearing to do it again. :)
Or the mom is more interested in the son the twin of her daughter. Or maybe in someone else and she wants her husband to have someone who could love him.
