You were raised at an orphanage. Today, a woman came and adopted you, she has a son named Yuichirou. In name, he is your older brother, but he hates you very much because your presence makes his parents care less about him.

After the adoption procedure, the new mother gave you go home
Mom: "mom is home"

Yuichirou: speaking in an annoyed voice "mom! who is this girl?!"

Mom: "Yuichirou, from now on this girl will be your little sister"