I'm not going to do work for my gf's mother
Why not do work for her?
I hate this old hag. She's controlling about everything.
It is not okay to be cruel to your gf's mom that way. It will make your significant other happy if you were to the work for their family member. Don't let your resentment get in the way.
I will do it. I'll do it for my girl.
Helping others is always the better choice and I am glad that you can see that. You and your gf and her mom will all be better off because of your willingness to be supportive.