When it comes right down to it, it's HER house, and if she (with her husbands agreement) doesn't want kilts in the house, that's her business. So don't wear a kilt there. If that means not going there, then so be it. I would talk this over calmly and rationally with your wife, though. You're not terribly "responsible" to your MIL, but your wife is another story. You don't need to fall over yourself to do exactly what your wife wants on every single topic, but you DO need to treat her with respect and love and listen to what she has to say.

The rest of the time, wear a kilt if you want to. If your MIL is coming over to your house and objects to the kilt, I'd give her both barrels and not be shy about it....it's your house, you'll wear what you want, etc. etc. If all of you are going out somewhere NOT her house, wear what you want.

I am all for being reasonable and getting along with the relatives. I'm also entirely in favor of following the household rules when you're a visitor. I'm ALSO entirely in favor of standing up for yourself, loudly and firmly and repeatedly....though not RUDELY...when pushed.