Coffeeandbooks, what you are dealing with is so difficult and so destructive to everyone in your household. You are doing the right thing to cut ties with your sister. You have the right to make decisions on what is best for you and for the sane, beloved family in your life.
You do not need to feel guilty. Your sister is living out the consequences of the choices she has made, and she, and only she, deserve those consequences.
The truth is that no of us can really ever live someone else's life for them, even if we feel that we understand what they should do to be healthy better than they do. It is their life; it is their dignity and right to make their own choices whether we think they are good choices or not.
Maybe you could save the rent she gives you for the next few weeks before you make her finally move out. Then you could give her a check to spend on renting a new place for herself. You'd be sending her out own her own knowing that she has some financial resources, and that might be a more palatable way for you to end this.
ShootingStar1