I think kidd especially they pick up habits and behaviors from those around them. So if everyone is saying she's xyz it's easy for him to pick it up and believe that too
It happens.. i have seen in real life case like this. Mostly reason is parents. A child act as they saw their parents. Back in junior school i have a classmate who has little dark in skin colours but her whole family is fair. They feels shame calling her own child. I have heard rumors that his parents treat her like servant and don't give her any pocket money.. on other had they treated their brother normally. So just by seeing his parents he act similarly as parents and Don't treat her well..
I never understand how a sibling can blatantly hate their older sibling when that one has done nothing to you.