Something along the lines of, "Mom, you probably don't realize how much you hurt my feelings with the 'pants' comment, but I'll make a deal: I'll wear pants for Thanksgiving dinner, if you (and the rest of the family) will keep you mouths shut and not make any comments when I wear a kilt to Christmas. If you can't do that, it shows me that you don't value me as a person, and you only value me when I'm your puppet."

Lay it out plain and try to reach a compromise. If they're not willing to work with you after that, they're probably not the sort of family I'd be wanting to spend time with, anyway.

Brian