There is a contigent who believe that the first loyalty belongs to your spouse, not your child. I’m of this belief, but not when the marraige is dysfunctional and there’s abuse. Then, I think the children should be the primary concern. But not everyone holds that standard, and there are women that do choose the abusive husband over the children.
My mother’s one of them. My real father is the king of jerks, and she always has and always will choose him over anyone on earth. My advice is to try to come to accept that your mother has the right to decide this for herself, and it’s unfortunate for you, but you probably aren’t going to change it, so maybe you should learn to accept it. Make some new friends, get out of the house more, and try to make piece with it as best you can. Some things you cannot change.
Work with things how they are, and how they’re likely to continue to be. Not how you wish they were. You have no control over her, or him, only yourself and how you react.
I do hope you have a good time with your mom and other relatives, and don’t let him bother you too much. And Happy Birthday!