It's so annoying, but I have to deal with it?

okay so, in grades 7&8, my best friend and I hated this one girl that was a year older than us that was on our bus (let's just call her Brianna). Brianna is just so rude, annoying and obnoxious. she always called my friend and I annoying and immature, when she was the one doing all the immature stuff. to get to the point, during the summer of grade 8 to grade 9, I started talking to her and we became somewhat friends. now my best friend is all mad at me and thinking Brianna and I are all 'best friends' now. and plus (the thing that really annoys me..) EVERYONE thinks Im trying to be like her. My sister, my best friend, HER sister, and her brother are all saying Im 'best friends' with her and trying to be someone Im not (trying to be like Brianna). Ive tried standing up for myself and telling them Im not, but they dont care, they think they're right. I know Im not trying to be like her and it's starting to really piss me off. I told Brianna about it and she told me to just forget about it if I know Im not trying to be like her. but recently, I found out that Brianna too, thinks Im trying to be like her. Im so annoyed right now by everyone. I thought Brianna understood, but apparently she doesnt. I dont talk like her, I dont dress like her, I dont do my hair like her, IM NOTHING LIKE HER. But apparently, Im so good at 'trying' to be like someone, that I dont even know if I am doing it? they think Im a poser but really, Im completely myself. It's just really pissing me off, and I dont know how to prove to them that Im myself. personally, I dont think that I should prove to anyone who I am, but I just dont feel like dealing with their comments on how Im a poser and trying to be like Brianna. like piss off? what should I do? thanks for reading this really long description :P aha thankks again!