How about a story where a guy finds out that his lesbian sister has a crush on his girlfriend? He doesn't take it well he thinks she'll try to steal his girlfriend from him. He ends up having sex with his sister, either through rape or she did it willingly to convince him she wouldn't steal his girlfriend. That or she did it with him in order to be able to also have sex with his girlfriend. Anyways his girlfriend walks in on them doing it and she pushes him off his sister. Maybe they make it a three way relationship or she breaks up with him and dates the sister or she leaves them both.