my dad. He quit drinking when I was a teenager, and we had a few good years, but he did eventually relapse. He's (I think, since I don't see him too much) in a state now where he keeps his drinking more or less under control but does do a fair amount of it. More than any of us would like.
With him, the biggest issue is that he undergoes a nasty personality shift when drinking. So really any amount of drinking is too much ... unfortunately he continues to do it. He becomes mean, abusive, defensive, and jealous when he drinks even three or four drinks. I don't honestly know how often he drinks these days but I've gotten to the point that I don't care that much.
Here is how I deal with him: I know it might not be helpful for you, but I try to be in his presence as little as possible (usually limited to short visits over christmas, etc.) When I am in his presence, I schedule my time so that I don't have to be sitting at home with him a lot. I try not to assume I should be able to hang out with him for long periods, since the state of his drinking and his mood are so unpredictable. When I am visiting, I make sure I have an "exit strategy" -- I make sure I have a rental car, have contacted friends in the area to escape to if needed, etc.
The other strategy I use which has been really successful is to continually remind myself that I am now a mature adult who doesn't have to put up with any bulls**t. I wouldn't let anyone act cruel/abusive around me, including my dad. When I was young, I was trapped with him and was helpless. Now that I am an adult, I can help myself and remove myself from the situation if necessary. I can leave at any time. I can call the airline and get the next flight out. This really helps me feel strong when I am around him. I have not had to leave his presence, but I have threatened to do so on a couple of occasions. This helped keep him in line and remind him that my presence is contingent upon his behavior.
I know you probably have to help take care of your mom and can't just absent yourself, but you CAN remind her that you will not be willing to help her if she doesn't act appropriately and treat you right.
Hope that helps.

Its a tough situation, and there is no easy solution, but just remember that you are a strong person who is able to protect yourself from harm. Being around parents sometimes makes people feel childlike and vulnerable -- but you are NOT vulnerable. You are going to help your mom because you're a strong, mature, caring person -- but you're not going to let her hurt you.