Time Traveling Relatives

There is a young man in his early 20s. While going to college he's still living at home with his mother. And that is kind of embarrassing to him. But only a little. Have him be the main character. He would have a half sister who is anywhere from 2 to 5 years older than him and just finishing up college. He has the hots for his sister but she turns him down whenever he tries something. The sister goes to a different college and doesn't live with them. While the sister is away their mother says that her long lost niece is coming to town. The new relative looks a lot like both his mother and sister. But she is only about his age, maybe a half to a year younger. The boy is sexually frustrated that he easily falls to her seductions. We eventually find out that the girl was either his mother or sister brought there from the past. We find out that the girl ended up getting pregnant. So if it was his mother we find out that when she was sent home she lost her memories of the events or they were changed in a way to have her think she ran away to be with a boyfriend or was kidnapped. And we also find out that his older sister is his daughter. If it was his sister that traveled through time we find out that she had been sent away for a year, so she could keep the pregnancy from her brother. Either way the story goes after the sister finished with college she has sex with the brother and gets pregnant.