Help me with my jerky "friend"?

my godmother's daughter is a total pain in the behind ever since ive known her. When we were 4 she would throw sand and pinecones at me. But lately she has been spreading fake rumers to my cousin about me and she is always insulting me! I hate her! she always makes bad choices and leaves me in the middle of them too. shes at a counsiling school thingy for now for 100 days and she ran away so shes going to be there longer. but i know that once she gets out we will HAVE to hang out. Ive told everyone how mean she is but my mom still has us hang out. I dont know what to do. Sometimes my mom will make excuses to make me not have to go but when she gets out of her counsiling school they will come over for dinner and shell sleep over, wreak my room, and comment all my clothes. i have no backbone so i cant stick up for myself and when i have a friend over too so she can woop her ***, she becomes a little angel. she always calls me ugly and makes me feel insecure. So how can i give her a littl