Why are black girls so vicious?

My sister has short hair and dreadlocks and I know next year for high school plenty of black girls are going to be the ones making fun of her. Why are black girls so vicious? They are always the ones making fun of how someone dresses, how so & so has no , how so & so is ashy-as-hell, how so & so is as dark as night. Geez. It is irritating. If you avoiding hanging around with black girls, they think you're a sellout or that you feel you are better than them.