J.L.
Hi L.,
My mother in law and I don't get along. She does not think I am good enough for her son. This bothered me for a long time, but now I just think of what my family says and my friends, and remember that I am a good person. I am polite to her, for my husband's sake, but am not friendly.
I would go to the party, as much for your husband as your daughter.
I also wonder, and I don't mean to sound rude, but maybe she does not feel included in your duaghter's life. She does baybsit for her, and lucky woman that you are, and maybe she wants to feel needed. Maybe to appease her, ask her opinions on something or ask her if she notices your daughter doing something.
But be the better person, be polite and do what you can.