Here are a few suggestions:

#1 - Agree to never wear the kilt to the in-laws, and at each invite, state that you won't be "comfortable" going to the in-laws and decline to go. Besides, you're too embarrassed after the last visit.

#2 - If your anatomy is able, wear jeans or trousers with a waist a bit too large for you, so that you show a bit of "crack", and then pick up a habit of being a bit clumsy; i.e. "Oops, I dropped my napkin again. Let me just bend over and get that." :butt:

#3 - As was stated before, wear that dang kilt everwhere except the in-laws, and be sure to wear it in every photograph that they'll see, especially Holiday cards!

Seriously, she may be able to control what happens in her house, but don't let her control the rest of the world. Try to be amicable and work out something that everyone will be comfortable with, including YOU! Keep your chin up!