I'm thinking the sisters father is Yuu's father or his mother was in some way relatd to their mother. It would explain why he took Yuu in.Well the thing is it felt so onbrand for Sakura to do that. Also, I am still hoping for the author to reveal Sakura is Yuu's sister because Sakura only live with her father from her childhood with no mother in sight and Yuu (formerly) live only with her mother. Therefore, making it an incest route with Sakura.