Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!

Gnomish

fwîr

noun. niece

The word G. fwîr “niece” appeared in the Gnomish Lexicon of the 1910s, a combination the feminine patronymic G. fwi- with the Gnomish feminine suffix G. -ir, so properly meaning “sister’s daughter” (GL/36). It is likely related to ᴱQ. súyon “nephew” (QL/87), and arose from something like ✱su̯īr > swīr > fwîr with the (Gnomish) sound change of sw > fw.

Neo-Sindarin: For purposes of Neo-Sindarin, I would adapt this word as ᴺS. hwiel, with (a) the Sindarin sound change of ancient sw > hw and (b) the later suffix -iel “-daughter”. Since we have no other words for “niece”, I’d use it for female children of any sibling, not just a sister’s daughter.