Best friends gf

My bestfriend recently started dating this one girl, an she absolutely hates me! She thinks that there's something going on between me and her bf when there isn't a thing! My bestfriend, who I will name as "B" is like a brother to me! We met when I was 16 and he was 22, he's really good friends with my and my dad introduced me to him. Ever since then I have always referred to B as my big brother and I have always been his little sister but since he started dating this girl we've been growing distant and it kills me! The one night I was having a mental breakdown and the only person I've ever opened up to was B and I was talking to him about my problems and she pulled him into the other room and told him how she wasn't comfortable with him talking to me as much as he had been. So when  they came back he sent me a text asking that we all talk as a group and it really pissed me off, cause they had only been dating for two weeks and it just isn't fair that he should listen to her and I feel like she's driving a wedge between us. 
I've been trying really hard to be her friend but she's just not putting any effort into it and I can't afford to lose B because of her... 
I have no idea what to do! Any advice would be greatly appreciated!!