Did your younger brother or you and your older brother ever seek out Al-Anon or any of the connected programs?
It sounds as though that was not a choice for you diseased mother.
In my case, as my husband downed a 6 pack of beer every night, I took the advantage of attending Al-Anon and took my kids to Ala-teen.
Would have been pointless. There was a great deal more to this than alcoholism. You did notice I said "mentally ill"? To the point it looked like demonic possession?
I did what needed to be done. As did my older brother. We got the hell out of there as soon as we got out of high school.