Personally, I think the best way to help her cope is to be a friendly ear for her.
I don't know what more you can do other than that. It is her responsibility to find healthy ways to deal with her mother's alcoholism, and her families dynamics.
Sometimes when I am needing my friends, I don't need for them to fix anything, and they CAN'T fix anything, but I just need them to be there to listen when I need to talk. That's what friends do. JMHO.
Don't take on stress in your life that is not your stress. Just be a friend. JMHO
Good luck
