Please help me!!?? :(?

im so upset, angry and confused about what to do! :( (LONG story short!) my boyfriend is my friends brother. after about a month of going out with him i was asked by the "popular" girls who he was. before i could answer his sister shot back and said "hes my brother" and she would not only bring me down about the fact he is my boyfriend infront of me, but also behind my back. we have had 2 major fights about this and i think she might be spreading rumours about me and him (like being pregnant etc..) and the "populars" always say stuf like "wont it be abit awkward at the year 10 formal when both you and him are together when so is his sister" (and i always feel like im doing wrong?????) i am thinking of moving schools because of how bitchy it is (all girls school, typical!) and also because my best friend has changed alot and she is a major sheep! she follows everything every other girl does but especially one of the girls in my group. it really gets on my nerves because she always makes me feel like shes moved on from our friendship. 2 girls in particular have talked about her to me in a derogative way and i told her. but i feel like they are talking about me behind my back to her but she isnt telling me! :( i am so sorry its long and not very in depth but i hope i gave enough information to help you help me about what i should do?? please. i really appreciate any advice as i only have 1 more term before i have to make a final decision!! :(