G4life, this is serious, and if I was in your place, the only quick thing is to get her out of there, explain to the teachers and faculty why, get anything with your complaint in writing if possible. But #1, remove her from the threat. Since your the older brother, I don't know if your 16 or 30, matters a little bit, but if your underage, you still have avenues. Your sister not going to school and documenting why is an easy 1st step. If you or she documents each step, it will be easier for later, lawyers, police etc... teachers and administration don't like the lawyers or police. If challenged, with authority, they will usually back down quickly.
Get her out of there and talk with your parents or her guardians seriously, but don't let her back in until something positive has been done.
Best of luck