It was similar for me. I went out with my ex's brother a couple of times before we got married. His brother is a cronic alcoholic too. Lost 2 wives and I cant believe he is the man I knew 30yrs ago. I cant recognize him.
Unfortunately my sister will probably end up like your brother in law. It is such a terrible thing to watch - someone slowly killing their soul & body. The way they punish & hurt themselves.
hugs JJ