For more ideas, and NSFW pictures, GO HEREIdea 1:
-Sarah sneaks out at night to party with her friends. Sometimes she comes home stoned or black-out drunk. Lately, she has been mistaking her brothers room for her own, passing out in his bed right next to him. He was a little put out the first time, and a little miffed the second time, but the third time - well, he decided to see just how passed out his sister was. When she didn't wake as he undressed her, he started to touch her in inappropriate places - and then started to rub himself against her. Of course, why not sate some of his own desires and kinks? So he starts to fuck her too. Sometimes she is more responsive then others, sloppily sucking his cock or even riding on top, but most of the time he hammers away at her however he likes. He has even started recording their sessions.
But her older brother is perverted and knows their younger brothers would love to get in on the action too, so one night he invites them in and shows them what to do. They record the session and use the tape to blackmail their party-going sister into doing sexual things with them when she isn't stoned or drunk!Idea 2:
Cheating Wife - a wife cheats on her husband with her boss, performing sex acts with him that she would never do with her husband. Her husband slowly catches on and finds himself more aroused by seeing her with another man, and their sex improves because of it. So he starts sharing her with other men - coworkers, his neighbor etc.