I've never done any RP of this kind, and I'm sort of interested in doing one.
I would rather have a FxF pairing, possibly mother/daughter, or simply Sister/sister.
For a plot I was think this could either be 1x1, or possibly with 3 people. The two following plots are slight variations on the same idea. For both ideas, I would rather play the daughter, or the younger sister.
The mother is caught cheating by perhaps a family friend, or a private detective, to keep it a secret, the mother agrees to do whatever the man wants. Thinking that he simply would have sex with her. However the man forces her to practically rape her own daughter, on a number of occasions. Each time the order getting even more obscene as time goes on, as the mother is forced to both mentally, and physically scar her own daughter. Practically turning her own daughter into her sex slave.
There would be some age play involved probably, bondage, and whatever else. This could be a three player game by having one person, as the wife, and another as the blackmailer.
The older sister is blackmailed, possibly being caught stealing, or cheating on a test. In return for not turning her into the police, or to the school, she is forced to do terrible things to her younger sister, who doesn't know the reason why her usually kind sister is doing these kinds of things to her.
Basically the same idea, probably college based, or possibly high school, but I'm assuming that 18 is the minimum age.
Or possibly a more romantic avenue, but contact me via pm regardless.