I feel a little bit helpless, because I really cannot guess, who this nephew may be.

The youngest brothers, this would be the Ambarussa, but tjey are told to be unwed. 

The word fair could be a hint to Celegorm, but he also did not father a child.

Curufin only had one son, and he knew Maglor well.

So again, who is he, brother of a fair sister, and himself dark?

I feel a little bit helpless, because I really cannot guess, who this nephew may be.

The youngest brothers, this would be the Ambarussa, but tjey are told to be unwed. 

The word fair could be a hint to Celegorm, but he also did not father a child.

Curufin only had one son, and he knew Maglor well.

So again, who is he, brother of a fair sister, and himself dark?