The word G. fwion “nephew” appeared in the Gnomish Lexicon of the 1910s, as a masculinized form of the feminine patronymic G. fwi-, so properly meaning “sister’s son” (GL/36). It is likely related to ᴱQ. súyon “nephew” (QL/87), where *su̯ion > swion > fwion with the (Gnomish) sound change of sw > fw.

Neo-Sindarin: Since ancient sw > hw in Sindarin, I would adapt this word as ᴺS. hwion, and would assume it is a now-obscure elaboration of -ion “son”. Since we have no other words for “nephew”, I’d use it for male children of any sibling, not just a sister’s son.