I don't know what to do about my brother in law

My brother in law has been staying with my family for almost a year now. For backstory, he was in his early 20s when he lost his mom during COVID. He was working at the time but quit due to major health issues. Then his dad developed major health issues as well, multiple hospitalizations with no help from his doctors, developed dementia and unfortunately passed away . My BIL was his father's sole caretaker, 24/7 around the clock, and witnessed firsthand his rapid decline into nothingness. He developed an addiction to alcohol and quit cold turkey due to liver damage. He was so close to death himself and has major unresolved mental health issues. At one point he was experiencing hallucinations but now it is my understanding that he just has major depression and extreme OCD. He is on anti depressants however he is smoking weed 24/7 to the point where he has built up such a tolerance for it that a large amount hardly does anything for him, and it interferes with his medication and any chance for improvement. He refuses to see a therapist. He refuses to look for work because he thinks he won't mentally be able to.

He was living in his parents house with no job living off his grandmother's inheritance (she was the third to pass away), up until last year when the oil heating tank leaked forcing him to move in with my family. Because he was living without heat before all that happened, the pipes froze causing more damage which we still havent been able to get around to repairing 100% to make the home liveable. It will not be liveable until probably the spring which is when I strongly suggest we sell it. I suggested selling it a long time ago but he and my husband wanted to hold on to it because of the memories. If it weren't for the inheritance we literally would not have been able to pay for it as well as our own mortgage. Additionaly the mortgage is paid off but we have to pay around $30k in inheritance to my BILs (per the will) so that would come out of our pockets unelss we sell the house. It literally does not make sense to keep it.

So in the meantime he has been living with us for the past year. Refuses to help himself and is very combative whenever my husband brings anything up. Wakes up after noon and smokes weed all day/night. We have a toddler so I made him smoke outside but the smell still lingers sometimes, which I don't really like. He's constantly in and out the door/up and down the stairs at night which bothers me but I just deal with it because I want to avoid a fight. He is so quick to blow up over the smallest things and when he does he completely shuts down and hardly talks for several days.

He's in a very dark place. I know that any change is entirely up to him, he has to want it, but I don't see that happening. My plan is to sell the house and use that money to buy a bigger home where we can all live together comfortably, maybe a separate unit for him, but essentially we would be his caretakers.

It's just a really unfortunate situation, I feel so bad for him and what he went through and want him to get better but he doesn't feel like he deserves any of that. Honestly if I were in my husband's shoes, I would give him an ultimatum, that he needs to find a job and a therapist as terms for living here but to my previous point he can only decide that for himself, and I think if he did that it would push him over the edge. I constantly worry that one of us may find him dead. I just dont know what to do, or how to help, and need outside advice.