I'd say Jock is spot on. Also, I assume that your sister is expecting you to wear your kilt, but how about the other two weddings? You say that the second wedding will have lots of national dress from different cultures, so the kilt should be fine, but I'd leave off the fly plaid myself. As for the third wedding, you might have your girlfriend inquire of the bride about your wearing a kilt, no sense in "rocking the boat" as it were.