"What is the name of your twin?"
Translation:Vad heter din tvilling?
I don't think that's correct in written Swedish, but spoken it would work. In a similar manner you could have said "tvillingen din, vad heter han/hon?" It looks odd written, but spoken it would work. Similar to "that twin of yours, what's his/her name". I don't think that's how it's usually written. But spoken it works. :)