Don't stress about the family function. Go ahead as normal, then feign a stomach upset/gastro. Have you Protein drink in the hotel before you eat out then settle for a peppermint tea, "to settle your stomach" when you are out with everyone. If anyone really pushes you on it, tell them in confidence you have a stomach ulcer and can't handle anything too much to eat.
I'm having surgery on jan 21 then it is my sister in law's 50th on feb 1. My "stomach ulcer" is going to be playing up that night!
Good luck. Fingers crossed most people don't even pay attention to what we eat or don't eat.