My personal opinion:
I wouldn't break my hubby's mother's heart even if she's being irrational.
So you would rather please your MIL than hubby? And tell him to deal with it, since we are only talking about wearing some clothes of her choice?
See i can't do that, don't get me wrong i do respect my future MIL to be, but if she tells me one thing and my hubby tells me another, i'm gonna go with my hubby, unless of course, he wants me to just do it for her. I think this relationship needs to develop early between women.
I'm not married yet, but my soon to be MIL, knows i'm not gonna do anything without my hubby to be there, and that if i'm not happy he will take care of it in a nice manner. And this makes it sound like i hate my MIL, she IS crazzy but i love her, she isn't that crazzy that she would stop talking to me becuase I didn't wear what she wanted me too!