For BIL's sake, someone needs to convince his mother that her 'treatment' is a very poor idea.
Is there any way you can convince her to read something written by an expert? Watch an episode of Intervention? Attend a meeting for friends/relatives of substance abusers? Sit her down with a recovering alcoholic?
BIL is in big trouble, and he's not going to get out if he has his mother telling him he's doing the right thing. Yes he has to make his own decisions, but being undermined in good behavior and supported in negative (for him) behavior by a parent is a recipe for disaster.
Would your father be willing to talk to her about the dangerous road she's taking? Sometimes the only way to wake someone up is with a very cold dose of reality from someone who's been there, done that, and worn the tee shirt. Failing that, would he be willing to have a talk with your BIL? After all, at twenty he's an adult and needs to be treated as one. If someone can help your BIL see the light, he may find the strength to stand up to his mother and her ideas for 'curing' him of his alcoholism.
BIL is going to need a lot of help cleaning up his act, once he decides to do so. If his mother won't listen, maybe someone should start talking to him.
I understand why you don't want to involve your family history, but sometimes you have to swallow a bitter pill in order to help someone else.
And in the end, you have to let everyone else make their own decisions, for good or for ill. That's what the serenity prayer is all about: accepting that there are things you can change, and things you cannot.
Best of luck to you and to your BIL. You're both going to need it.