What a difficult thing to deal with. Kudos to you for helping give your sister's children some stability. They really need all the love and support they can get right now. As for you, take care of yourself and your own family right now. Her choices sound like they have a toxic effect on you. Let her go, disconnect. Don't let her drag you down into her vortex of destruction. If she stabilizes and if it feels right, by all means allow her back into your life some time in the future. And try to watch out for her children as much as you can. They might be on the same destructive path she is. Good luck!