Why oh why hasn’t your family been in therapy? Obviously, this isn’t okay. Your sister’s behavior and your mother’s fear of her isn’t at all normal. You are carrying many diagnoses that need treatment. This has got to stop. If you or your mother could end it, you would have done so long ago.
I hope the family hasn’t been “protecting” your sister by not involving other members of the extended family who might have been able to help you. You didn’t mention your father. If he is in the picture, he might be able to help as well. Ideally, your sister would be removed from your home and provided with therapy to deal with her apparent need to victimize her sister.
Your family needs family therapy. If they won’t participate, at least get yourself into therapy. You need and deserve the guidance and support a counselor can provide while you get ready to leave home.
If you are already in treatment (and I certainly hope you are with all those diagnoses) but haven’t talked to your therapist about this situation, please share this letter with her. Your therapist needs to know that you are living in an abusive situation day in and day out.
Meanwhile, prepare now to leave home as soon as you can. Do well in school so you have a chance at scholarships. If that isn’t possible, get a part-time job and learn skills so you will be able to get out of the house and support yourself.
I’m so very sorry that you haven’t had the close twin relationship that is so often the case with multiples. You and your twin should have had help with this situation long ago.
I wish you well.