Translation:A nephew is the son of a brother or a sister.
I think you have a point. I contracted the sentence even further than the suggested correct answer to simply "...of a brother or sister." Wrong apparently but I still think my answer is more correct being a native English speaker. I suppose it is difficult to get the imbalance between literal and pragmatic translation.
Is there any way to distinguish a grandson from a nephew in Italian or is there only one word? While I love all of my nephews dearly, I got my first grandson a year ago. I'm visiting Italy in a couple of months and I will need to find someone to brag about him to. And I want to make it clear who I'm talking about...