Boku wa Kimi no Tori ni Naritai.

Koyama Kei lost his mother when he was still in elementary school and never connected again with his distant, workaholic father. The only real light in his home life is his sister Moe: a bright college student who obviously adores him. But Kei has intimacy issues that even his sister can't pick up on. 15-year-old Kei has never been in love, but he has sex with anyone who shows him interest. While he's in someone else's arms, he can forget the void in his own life. One day in late Spring, Koyama discovers what's missing: a slightly spacey med-student named Fuji. Unfortunately, Fuji is his sister's boyfriend. (Source: DokiDoki/MU)