Should I end a relationship with my husbands mom?

No. Just don't let her take the kids anywhere and tell her she has to be sober and kind at your house when she visits. Tell her you are worried for her and suggest she enter a rehab program or seek a doctor's supervision for her mental health and chemical issues. Try to help her. She's family. If she never gets better or continues to be a danger to your family, dump her like a hot rock.