I'm so sorry to hear this. I hope the situation gets better for your brother and Mom. As for your sister, I think you should all try to get her into therapy. It's expensive and difficult at first, but most find it to be very helpful. Find out why she's acting like this, mental illness is scary and hard to deal with alone. Ignoring the issue will only create more problems. Communicate that it's unnaceptable and that she needs serious help and possibly medication. I know you didn't ask for advice, though, so I'm sorry if this sounded demanding or pushy. It just breaks my heart to hear that she's hurting your mother and doing this reckless stuff. Unchecked, this could ruin more lives than just hers. Wishing you the best.