Here's a possible set-up.
Matthew and his younger sister, Louise, go to a party. Louise is hot but sexually naive. During the party, she drinks more than she should and she goes upstairs to lie down in an empty bedroom. Matthew later finds her on the bed, but she's not alone. - a guy has his cock in her mouth. Louise is clearly out of it, but she seems to be sucking the cock with vigour. Turned on, Matthew snaps a few pics on his mobile phone. He even gets a couple of shots of the guy cumming on his sister's face. Of course, Matthew could have a go himself, but he can't chance being caught fucking his own sister's mouth at a party. He later uses the pictures to blackmail his sister into becoming his sex slave. It's bad enough that she's sucking a guy's cock, and seemingly enjoying it, but the guy in question is black, and Louise's Dad is a racist, especially after his wife left him and ran off with a black co-worker. Poor Louise will do anything to keep her Dad from seeing those pictures, because she knows he'll never forgive her.