Here I am, filled with grim resolve.

I have a history of problems with booze and other substances myself. I stopped in time, before becoming a full-fledged alcoholic. Now I'm watching a family implode from within. It's bad, bad, bad!!

An intervention is the way to go. If the key player isn't on board, my only other option will be to talk to the police / child protection about exactly what has happened in that house. Yep, I'm going to use blackmail, if necessary. No matter what I do, there will be reverberations for years. If I do nothing, somebody is going to die. I'm not being dramatic; it really is that serious.