Hi, Matt
For starters, I want to extend my strongest and whole-hearted sympathy to you.
I also want you to understand that yes, this is a real problem, and you have every right to feel the way you do. I also believe she does have a mental illness that needs treated and she needs to be watched very closely-- perhaps medication is not enough to help her.
Also, if she is threatening to hurt herself or commit suicide, it is not a bad idea to call the authorities and report it. Maybe that is the help she needs and is secretly seeking-- maybe being admitted somewhere to be monitored closely will help her. And once her condition is stabilized and under control, maybe then therapy will help her realize what she has done to you and continues to do to you is wrong and inappropriate.
However, I do not think you are in the wrong for not being able to forgive and forget-- this is abuse and it is a difficult thing to live with and understand that it is happening to you. I think it would benefit you greatly to receive therapy to help you through this difficult time. The therapist may be able to help you understand things you cannot do by yourself yet and help you in ways I may not know enough about.
I do know that you have every right to feel the way you do about this situation, your family situation, dealing with the abuse and trauma from your sister, and not knowing what to do next. Please remember that you are only human, and you can only do so much. You also should remember that you need to worry about you most. Worry about your well-being, your safety, and quite frankly, your sanity.
I wish you the best.
-AmberB96