Kat, oh my God, I'm so sorry! What an absolute trauma, nightmare--! Please take really good care of yourself. Don't get sick!
Find out as much as you can about the laws where you live regarding child protection services and domestic violence. If there are under age children living in the home with domestic violence going on, then the pressure of the kids possibly being taken into custody can at times galvanize the mom to leave the boyfriend who is abusive, so that she can keep her children. Find out all you can about the laws regarding hospitalizing people with mental illness. If there is evidence someone is a possible threat to another, he can be hospitalized in a psych unit against his will. Sometimes this can help get someone away from the person he may harm, get him somewhat stabilized on medication. These are just my suggestions from working in the past in the mental health system in the U.S. You may as a family have more power than you all realize, if you can keep in touch with the children, keep getting information, and keep communication open with law enforcement.
I'm so sorry, I can completely imagine how'd I feel in your shoes. My thoughts are with you. xoxo
Jan