Bran....
Im sorry you had to find us again but glad you are back. This may sound tough but....put her back in the system and then if she signs herself out at 18...let her go. It is her life and she has to make her own decisions now. Maybe in those few months she will hear something to make her decide to stay in independent living when she knows she cant come home again. If not, there are shelters. You and your other family members dont deserve to go through this anymore.
There is a book about borderlines called something like walking on eggshells. It also has a workbook to go along with it. Get it. I am also borderline and it is a tough diagnosis to deal with but it can be overcome but only if the person who has it wants to get better. I am in therapy weekly and I still have a long way to go but I am not out there trying to hurt people anymore. The only way to protect yourself from a borderline is to put up some strong boundaries.