Hey,
It sounds like you're in a really tough situation, and that you have been for a very long time. And it also sounds like you don't really have any good options right now - go with the abusive dad or stay with the alcoholic mom... That sounds like you're choosing between what's the least bad. And I kind of think you deserve more than that.
It also sounds like you're very protective of your mother and don't really have very warm feelings for your father? And that's OK. I mean, it's damn hard to love someone with BPD. They're not very lovable, in general. And if you've also witnessed him being abusive to your mother, so much harder to love him. But I'm just guessing here based on what I read - I'm sorry if I'm misinterpreting what you're saying.
You sound worried about your mom. I would be too if I heard someone say what he said about her. I'm not sure what it means either but it doesn't sound good.
My story is that I was married to an alcoholic for 20 years. I loved him. And I desperately wanted him to stop drinking. It was very hard for me to accept that I couldn't. That I didn't have power to make him do anything. I had thoughts like "if he loved me, he'd quit drinking" and "if I was just different, better, he would get sober." Our kids believed that if they just did better in school or didn't fight, he'd quit drinking. None of that was true but we believed it anyway.
I don't have any great advice other than that... No matte how much you love your mom, you can't make her stop drinking. But you can help yourself. I would go to a few Al-Anon meetings and listen to the stories of other people with alcoholics in their families and how they manage to stay same. I'd also recommend any books by Melody Beattie about codependency.
I'm sorry I don't have anything ore helpful tonight, and I'm even more sorry about the situation you find yourself in. Just know that it's not impossible to feel better. And I'm sure other people here with have lots of good things to say. So hang out for a while. This is a pretty good place for support.