Can’t stand my bfs sister

She’s treats everyone like shit bc her mom spoils the crap out of her. She keeps making fun of people for things they can’t help and when I told her it wasn’t cool she ran to her mom crying and now her mom is telling my bf to break up w me and he is PISSED at me he wants me to sit back and “ignore it” he goes off on people who hate his sister but never goes off on her for giving people a reason to