My brother's gf hates me?

This is hard to explain, but im going to give it a try. I'm eighteen years old, I'm finishing high school this year. I use to be a drug addict, but have been clean for a year and half now. My brother's girlfriend is 28. I also cleaned up my look and started wearing nicer neater clothing with heels etc. Since I started changing, she changed towards me. At first I thought she was great and I thought we were getting along so well. But then she subtly started ignoring me when we were with people. ( that thing people do to alienate someone they dont like). And as the months has progressed she became very competitive, watching my every move and trying to intimidate me. At first it bothered me a lot, because i'm one of those nice people, whatever, but everydays anxiety ebcame less and less and I felt like she actually helped me become more confident in who I am, because she loved it when I felt insecure, but now i'm much more sure of myself than ive ever been before. She also Hates me, I see it on her face, she ignores me all the time. She will speak to the person right next to me, but will flatly ignore me, if she has to look me in the face she gets more angry. I just decided that I wasnt going to lower myself so that she can prounce all over me! I have been working on becoming normal and adapting to this new lifestyle for so long and then she comes and tries to push my face into the floor. I wasnt going to allow it. And now my mom tells me that she's scared of me, and that she's actually very sweet and kind. But I cant seem to see any kindness in her behaviour towards me. Sometimes she's nice to me and other times she ignores me, wtf? either be nice or just **** off. Does anyone else have to deal with women like this? How do you deal with them? Do I really have to be nice to her just because she's my brother's girlfriend?