My brother and his girlfriend are trying to act like my parents!?

[Amanda B]

After my parents died when I was eight, my brother became my legal guardian. Just him, not his girlfriend. His girlfriend just starting spending a lot of time over at our house, and it's so annoying because she's trying to take over my whole life. She decides randomly to not let me go out with with friends, or that I'm speaking too rudely to them to be allowed to be in the living room and I have to go to my room. Then, if I bang my door, she comes and takes my door off. I yell at her for not having the right to take my privacy away like that, but she just snaps back and me and says that I have no right to speak to her like that. And my brother never even stops her, he just lets her go on trying to act like my mother! Help! What do I do to get her to back off?!