i had the same problem with my sister, she was a year below me, but seemed to always make everyone think she was better. The only way i could fix it, was by trying to be nice to her. Even though she was awful to me and told my friends lies like i was talking about them behind their back. It just comes down to one of you being the bigger person. Anyways, if your friends were your friends, they wouldn't fall for her act. They'd have you back.