"My paternal grandmother and my maternal grandfather come."
Translation:Farmor och morfar kommer.
Would it be wrong to say "mina farmor och morfar kommer", because you're talking about the two of them?
Saying ”farmor och farfar” is a bit more familiar and colloquial, I’d say. You’re more likely to say ”Min farmor och farfar kommer på lördag.” (My grandparents are coming on Saturday) than ”farföräldrar”. But you might say: ”Hans farföräldrar var båda lärare” (His grandparents were both teachers) when writing someone’s biography or something. But you can use either, there’s no big difference.
The best translations are usually the most "natural" translations. In Swedish you often omit "my" when talking about close relatives, but since the English sentence requires the words "maternal" and "paternal", "my" is needed here.
Adding the possessives is of course fine (you should not lose a heart for that), but it is not the "best" answer.