An anime/manga where the parents of the two main characters, Miki and Yuu, swap partners and remarry, and they all live under one roof. Miki and Yuu begin hating hating each other, then grow to like each other and go out, then think they might be related, then end up not being. Oh, and everyone plays tennis.
Friend: Tell me spoilers!
Me: The first word of book five is "I"
Friend: Now I don't even have to read book five. I can just guess what happens in all of them.