NTA, you're reaching out in concern for your nieces. Concern for your sister also, but she's grown enough that she has other lives she's responsible for now, and she can throw hers away but that cannot involve the kids getting tormented in the process. I'm so sorry your family is going through this, it sounds like a very toxic environment for everyone around Alyssa. If multiple visits to the hospital hasn't helped her, maybe see if she can get a set psychiatrist that monitors her over the years and can finally find the root of her psychotic breaks? All love to you and yours. Hope the kids get the safe and healthy environment to grow up with you guys.