Please help settle a dispute: Am i right to take offence and is this racist?

I'm Scottish staying at my fiancee's house in England and some members of her family came in and sat downstairs. Her uncle and her cousin had a joke about her having me in a box upstairs which ok is fair enough. Then when they started to leave they started banging on the walls going "let me out, let me out!", her mum asked what they were doing and one replied "Oh, Just saying goodbye o HAMISH!!", "See you later HAMISSHHHH! HAHA", "HAA MACBETH!". That's the way it happened exactly. My fiancee now isn't speaking to me because i took offense and it was "only a joke". I don't find that funny and i will certainly be downstairs when they come here again and see what they say. Am i right to take offense to this, would you? What's your thoughts? I know i shouldn't get annoyed by things like this but i don't feel like i should be spoke about like that. If i said anything back it would probably have resorted to violence. Then i would be the bad one.