Why is my boyfriends brothers girlfriend so rude to me?

I’ve been with my boyfriend for a year and his brothers girlfriend has never liked me. She doesn’t talk to me, rolls her eyes whenever I try make an effort, and tears me down when no one is around. She is nice to everyone else so I’m not sure if I’ve done something to offend her, or if she’s just an awful person. Any suggestions?
Why is my boyfriends brothers girlfriend so rude to me?
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