Ah Man that sucks. I think it IS a huge thing to have dumped on you, especially as you have serious vulnerabilities yourself. This would be right on the top of the therapist's Life Stress Indicators. This may sound tough, but I would (and I really mean I would, as I am this tough) contact his family and draw some boundary, some line in the sand so that you protect your own physical and mental health. Maybe tell them you can only visit him for a half an hour a day, or every second day, and that you can't be responsible for anything else to do with him. If you don't they'll keep calling on you to do stuff. You also fully have the right to withdraw completely from the situation.

I am both an alcoholic sober in AA 11 years and a Beeper and other illnesses. I know even just as an alcoholic alone, I can't allow myself to be dragged down by people who won't get sober, and where my life is at stake. And the same with mental illness - if others won't seek treatment, I can't allow them to undo all my hard work and again risk my life.

I know I'm tough on this stuff but one thing I've learned through AA and therapy is that I need to be assertive in my life and to have boundaries otherwise people will walk all over me.