Dump the girlfriend. Her family is hostile. It is not about the kilt it is about their ability to manipulate you. By the time I was in first grade I chose what clothes to wear, so should you. You are being insulted. Without the support of both families you are unlikely to have a long happy life togather. I can promise you this that the potential mother-in-law will do everything in her power to break you up. The hassle is not worth it.

Dump the girlfriend find someone else.