mirryben31 what a dilemma, there is no easy answer, my heart goes out to you.
I can only say I think you need to think of yourself. You are going through so much at once that you need strength and support.
Your mother presumably needs professional care/help beyond what your sister can do, and this needs sorting by whoever is responsible/able to do it.
It is quite reasonable for your husband to not want your son around after what happened. It seems you have to choose between them, but as you know, it is hard to help a person who will not help himself, even if are mentally ill. Do you have a priest, doctor, or someone with wisdom and experience to turn to?
Your son will not change. It is understandable that he will not sign on as he is unlikely to get work and the pressure and aggro will not help his mental state. I do not condone his situation but understand it. However, he needs to grow up and take responsibilty for his own life and not blackmail you with threats of suicide. Unfortunately he is unlikely to be housed by the council, the only possibility is a charity. Has your son/you tried any support groups like Mencap? However he may not qualify for assistance.
Think of your own life for the next ten years, do you want to share it and be supported by your presumably loving partner or do you want to sacrifice it all for your son and live with him, without your husband, somehow supporting him financially and looking after him until you are too old and weak to do so? If you do not meet his demands, and he were to go ahead with his threat, whose fault would his death be? (Not yours) Could you live with it?
If you want to avoid any upset before your daughter's wedding you could try to persuade your husband to let him stay until then on the VERY CLEAR AND FIRM UNDERSTANDING THAT IT IS ONLY UNTIL THEN but it may be very hard to then evict him without violence.
I can only suggest that you try Mencap, but they too would need him to sign on before they would be able to offer any sort of supported accomodation.
You are truly berween a rock and a hard place.