"What is the name of your twin?"
Translation:Vad heter din tvilling?
Tvillingbror would define that the twin is a brother, but the English sentence did not say that, only twin. So that's why I reckon tvillingbror isn't correct.
For a twinsister it'd be tvillingsyster.
Why word tvilling is in unknown form? Why it is not tvillingen, since it is defined by din and clearly a person does not have many twin siblings?
Been trying to find the right words to explain the grammatical reasoning behind this, I think I got it right, but I put some examples in at the end to try and highlight the difference.
Neither Swedish nor English uses a definite for the object when used in a sentence with a possessive adjective. "my the horse" just doesn't sound right in English, and it sounds just as strange in Swedish. :)
You could ask: "Vad heter tvillingen/hästen/kaninen" / "What is the name of the twin/horse/rabbit"
Which isn't quite the same as asking: "Vad heter din tvilling/häst/kanin" / "What is the name of your twin/horse/rabbit".
I hope that clarifies it a bit!