This must be a major issue and very confusing as a 14 year old.
I'm going to give you an honest opinion (aside from telling your parents)...
In cases like this, it is not uncommon that the child carrying out these behaviors has likely been a victim of sexual abuse so she may see this behavior as "normal,"... ESPECIALLY when the abuse is from a family member (even more likely both parents know have been informed and have ignored the issue). Many child abusers will often molest one child and not another in the family. Even if you were not abused yourself, your sister may have been. Also, you're cutting and started at age 12. You yourself may have surpassed memories (selective amnesia) yourself. It could also be a peer influence, or possibly being molested by a classmate? In any case, if your parents ignore the issue, call child services or go to her teacher. She needs help. Xoxo best wishes and hope I could be of help