My boyfriends sister 😤

My boyfriend has three siblings, two older sisters and a younger brother. They are all half Spanish half Mexican and I'm just a mutt of all European ethnicities lol. Anyways, when I first started coming around, his mom, his aunts, siblings, and anyone else you could think of was extremely rude to me. I figured it was them being protective over my BF since only 9 months before he had his heart broken by his ex of three years. And as time went by I've become extremely close to his mom, dad, and oldest sister. His brother and I have hung out and it's not that he's rude to me specifically but he's just an asshole as a person so I don't think it's anything personal. But the sister... She is the rudest bitch I have ever met. She won't look at me, talk to me, doesn't know anything about me. I've asked my BF why she doesn't like me and he says it's not that she doesn't like me but just like his brother she's a bitch. I can't help but feel it's Persobal with me tho. She walks around with her head held fucking high like she's above everyone. She was the ugly duckling and she suffered from anorexia. But nows she's beautiful, has over come her eating disorder and works at some high end job. It's like she thinks because she's done these things, everyone else is below her. Idk.. It's hard becasue I'm not a person to keep my mouth shut. But I would never do anything to hurt my BF and that's his sister.. I just ignore her right back cuz again I'm not one to kiss ass. I think once my BF and I have been together longer and she'll see I'm not temporary, I'm going to sit her down so we talk.