Here's an idea for a game I have. I played a similiar themed game a while ago and really enjoyed it.
Andrew was 17 years old and in his final year of High School. He had always been shy around girls, and this was his last summer before senior year. His parents had divorced years ago and he rarely saw his mom any more, preferring to live with his much wealthier dad. His dad had remarried, which meant he had a sister a few years older then him, home from her first year of collage. They didn't get along very well, but he had to admit she was rather hot. Was it weird to think of his sister that way? She was his step sister though. He spent a lot of time in the pool and pool house, where he stayed to avoid contact with her and her mother, whom he also didn't like too much. He would sometimes watch her through a window in her bikini, jerking off to her and her friends when she had them over. Little does she know but she has a crush on him, and always has, which is why she is mean to him. And now she's finally decided to seduce her younger brother.
Plot can be tweaked around as we see fit. Reply here or in PM.