Okay, I normally prefer to stay away from topics that tend to be emotionally 'hot', but I have to throw my 2 cents in on this one.
As I was growing up, my alcoholic father reminded me nearly daily of how stupid, fat and ugly I was and that NO one would EVER love me, least of all him. I was simply preventing his ability to have all the great things in life that he felt he deserved.
My mother married her sweetheart at 16, and had 2 kids with this abusive, alcoholic, two-timing husband. She didn't drive, they lived way out of town on a farm. He would disappear with any of the multitude of women for days on end, leaving her with two young children and animals to feed but no food or means to get food. At one point, she turned many of the animals out thinking they at least had a chance to find food for themselves.
I remember as a young girl, my mom wearing long sleeves in the summer time to hide bruises. He was smart in that he never hit her in the face where people could see it. He once sabotaged the car she and I were driving in. We were heading down the highway (moving) from Nebraska to Arizona after fueling up, and the gas tank FELL OFF the car. We know he rigged it because he had 'serviced' the car before we left, and assured us we should 'watch our backs' during the drive. He was evil, and we knew it.
When I was 13 or 14, I worked and saved until I could buy myself the one and only bathing suit I ever owned. One day when my brother was at work and my mom had gone to the neighbor's, he ORDERED me to put the suit on for his approval, NO underclothing, either. (This was the man whom I watched put my brother's head through a solid oak door and I was NOT about to defy him.) He immediately pointed out the bad things about the suit; too short, too low, too revealing. It was a one piece with a strap that went from one side, around my back to the other side, no shoulder straps. He suggested altering the strap slightly because if I didn't when I dove in, it would fall, like this........and he pulled the top of my suit so hard the strap tore, and the suit stopped at my waist.
I never wore the suit again after that.
I tried for years to have a relationship with him, he kept yo-yoing me back and forth, I was trash this week, but suitable last week. Even after I became a mom at 19, I tried for my son to have some sort of relationship with that side of the family. The first time my father referred to himself as "grandpa" to my son, I was done. I felt I was encouraging my child to endure the same abuse I had endured all those years. I wanted better than that for him.
The point to this long, convoluted tale is that I've been there, done that with the abusive father thing. I finally made the decision to 'divorce' that whole side of the family (suffice it to say the apple doesn't fall far from the tree). The second I did, I became a better person. I realized that I AM a valuable, beloved part of the world and that I AM worthy of everything life has to offer, just like anybody else is, including him.
11 years ago (Sunday, in fact), my father died of Multiple Sclerosis. I never went to see him (he'd moved back to Nebraska), didn't go to services, didn't send a card, etc. His family was mortified. I was then and still am perfectly comfortable with that. Had he asked for me, I'm not sure what I would have done. I really don't think I would have gone to him, but who knows.
Your father is abusing you and your mother, even your brother to some extent. At this point, he is obviously NOT going to stop. Your mother is more powerful and stronger than she realizes she is. My mother had no education, and wasn't even working when my parents separated (the last time). I went to work and was earning a whole $2 an hour, but supported the two of us AND my brother. We had a very patient and understanding landlord and found that honesty goes much further in some circumstances than money. We didn't qualify for goverment help, but we made it through on that $2/hour.
I think you and your mom know what the next step is, and I certainly understand your reluctance to take it, but know your own strength, have faith.
Whatever route you and your mom decide to take, know that you are not alone. There is a ton of support out there.