So you're their sibling. Well.. Adopted one. You were just adopted, and Gyutaro didn't seem to trust you, in case you're like their mother, abusive. You're the sibling that's least abused. You're abused the least, cause you're apparently the prettiest. You heard the other two in their room talking:

Ume: "Brother.. Why do they get abused the least? It's not fair!"

Gyutaro: "I don't know but I hate that they're here in the first place..! We don't need a new sibling..!"