I am so overwhelmed at the amount of caring responses

Thank all you so much.
I am leaving her alone for now. To be honest, my mother and I were never close. We are total opposites. She never really bothered with me much until my Dad got ill and passed away. My father did everything for her....cook, clean, shop, pay bills, drive her everywhere. She was very dependant on him. I do things for her because I love her, not because I like her. She was always mean spirited and sarcastic, but now more so. She never affected me, somehow I always have a strong sense of myself. I never feel that I have to win her affection. I guess I always thought that if she approved of me, I would be like her....that's not going to happen. She had her life being obsessed with my Dad and I had mine actually living. Now that my Dad is gone, she has no life because she was so dependant on him for 43 years. I have tried to get her to counseling, but she refused many times.
This episode kind of upsets me because it is disturbing to see a person behave this way. I guess I feel more sadness for her than anything -her grief has turned to extreme bitterness and anger. She has a big beautiful house, tons of money, a family that cares......and yet she has nothing.
My DH and I are going to go anyway. It's his B'Day and he loved it so much last year. My DH comes first. I will still take care of her and do what needs to be done. Unfortunately I will never feel any kind of closeness to her anymore due to one of her recent rantings. She told me that my father said to her as he way dying that "I feel so sorry that I am leaving you with her." I never believed it, my Dad and I were very close. It just shows me that I can only learn to be civil to her. Because of that comment I literally felt the love for her leave my soul.
Lesson learned: I did invited her because I felt guilty that we have fun and she doesn't anymore. I thought it would be something for her to look forward to. Don't do things out of guilt.