Sea Ray, I wonder if the fact that he is not married, lives alone, and has no kids plays into your brother-in-law's continuous cycle of self-destruction? His falling off the wagon just may be his way of getting attention.
I'm 40 myself and am not married and do not have kids. I know at times in my life currently the loneliness and the extreme quiet of an empty house can be very difficult the deal with. I'm a social drinker and have never dabbled in drugs, but I can see where it would be an easy trap to fall into as it can be an escape from the loneliness.
It sounds to me he needs to find something in his life that will motivate him enough to make him realize he is leading down a path that will eventually kill him. That "something" is different for every person, whether it would be finding a good woman, finding religion, or whatever. He has to want it though.
I sincerely hope he finds his way. Keep us updated.