How does this idea sound? A guy between his late teens and early twenties never knew his father and to do something special for his mother he enlists the help of his twin sister and his best friend who happens to be a girl and close to his age to help send him back in time. In the past his mother had gotten a love letter from a secret admirer and when she followed the instructions to meet the guy she didn't find him but ran into someone else, and that led to her having a NE night stand and gething pregnant withe twins. It would have been more but the guy died in an accident. She regretted never knowing who sent the love letter. So the son went into the past to figure out who sent the letter and possibly even save his dad, But he accidentally went too far by about a week or more before she got the letter. He ran into his mom and her best friend and they hung out for awhile. He found his mom fun but he started to fall for her best friend. He ends up having sex with the friend of his mother. He eventually finds out the friend is the one who originally sent the love letter. She knew her friend needed something to help her feel better about herself and also had some feelings for her, and had fully intended on showing up until she fell in love with the guy. He runs off to where his mom is supposed to meet his dad so he could see him once in his life and save him because his dad had died the night he and his sister were concieved. He could end up being the father of his sister and the mom ended up having sex with another guy that night or he was able to see his father but probably unable to save him and he didn't have sex with her. When he ends up going back to his own time line his mother is happier and he finds out he is the father of his best friend.