I can't stand my boyfriend's little sister?

My boyfriend and I have been dating for two years. We've been happy for all this time and rarely get into fights. I enjoy hanging out with his family, but he has a little sister about 11 he's 18 I'm 17) and she's a brat. I try to get along with her by playing games she likes, giving her some snacks and just doing my best to get over it maturely. She's been getting really on my nerves because she's started being rude to me. I really try to hold it down and I do, but I can feel the steam come out of my ears. Not to mention she just can't seem to take no for an answer, she talks back to everyone, and she thinks she's the boss. I don't know what to do. I've even contemplated breaking up with my boyfriend because of her and I feel awful for it. I don't know what to do, whether to talk to him, talk to her or just pull both of them aside. Please help.
I can't stand my boyfriend's little sister?
Post Opinion