I like KCW's suggestion of letting them know that you'll come in pants to T-giving, but at Xmas, you WILL be kilted. Gives them time to "absorb" the idea.

On another note, as others have said, there may be an underlying issue with your brother.

WIth MY brother, we joke around a lot and sometimes don't know where the 'line' is. Often I cross it and all it takes is a little frank discussion to put us back in each others good graces.

While I don't know your situation, may I suggest a direct, honest, heartfelt and candid discussion with your brother AFTER thanksgiving (don't even BRING the kilt thing up with him until after the thanksgiving meal... maybe the week after). Explain why it is important to you and that he's crossed a line. You respect the way he dresses, acts etc and don't try to change who he is or what he cares about. You would appreciate the same respect in return and mocking something you feel strongly about is not the way you want to interact.

If he can't respect you and have a serious 1 on 1 conversation, then you could learn a thing or 2 about how he thinks of you. Also, have a discussion with 'mom' after the fact to make sure she knows you're trying to take the high road. It's always good to have mom in your corner!