An issue with a girl.....?

Theres this girl at my school. Lets call her Sarah. Sarah always messed around with me and my BFF's lifes. She called us b****es and she was all telling her big sister and her big sister tries to bash us up. And she is really up herself by saying things like the time she just KNEW she would get an award (she didnt) and she said to me " oh goodie! Will you clap for me if i win?!?!?!?" and the next dy she will be all like gossiping to her big sister (I see them after school). Whats her problem?