My dad doesn't drink like yours, but had a time doing it. I was only 10 years old, now I'm 31, but I can still remember him arriving home and shouting, slamming doors, once he even appeared with a pistol telling me he would kill himself, and I was really scared.
Nowadays he is normal but you see, something was broken and I can never give him the same love I give to my mum. As you seem to love your mum a lot like I love mine, the only thing I can say is to be with her, enjoy the moments with her while you still live together, give her support, because maybe she just needs a hug and some conversation. It must be so hard for her to love a man who lost his own life to alcohol, she probably has tried many times to help him, but there's a limit for everything, and if HE doesn't want to stop it, nobody will. Pity is that she loves him so much she can't be tougher on him.
WE as children, can't do a lot, really.... unfortunately... so, like I say, stay close to your mum, try to forgive her for being such a soft heart, and let's think positive that when she stays alone with him in the house everything will be ok.
Good luck!