I think the literature and trying to get your MIL to go a counseling session with BIL is a good idea. Tough love is hard on the whole family, but it can save lives.
I have an Uncle who is in recovery. When his two (then) teenage sons got in trouble with drinking, dropped out of school, and though he would pay them for work and they wouldn't have to show because they where the "boss's kids - he kicked them them out of the house.
Now he paid 1st and last months rent on a room and board for each of them, he told them if they showed up and worked they would have a job (fishing and processing the fish so very hard labor). Some people in the family criticized him, because they supported him when he had his troubles. He politely told them - yes and you did it out of love - but I have no memory of my older three children's childhoods because I was drunk. I don't want that for my sons. (Both boys had fathered children - Uncle and Aunt did make sure both grandchildren were safe and cared for) Other family members did listen. They reached out to the boys, had them over for meals, encouraged them to go to Mass, but did not give them money, bail them out, or allow them to drink around family members. Both boys hit bottom and got sober, and are responsible parents now. It also sent a clear message to the younger cousins and we haven't had much of a problem with them.
I'm older than the boys - so I saw three Uncles (now all in recovery) go through downward spirals, and the death of a 4Th uncle. It is is a scary thing to watch when you are the niece - I can't imagine being the child. I took my Uncles decades to get sober. It took the boys a year to three years. I thank God my Uncle and Aunt had the guts to throw the boys out - or everyone might still be picking up the pieces after them - or they would be dead. I honestly believe Uncle and Aunt probably saved several other younger cousins (their nieces and nephews). They saw what happened with the boys, and made the decision to wait till they were adults to drink - or to never drink.
I honestly believe that some people in my family just can't physically handle sedatives including drinking.