Some time ago I was tasked to think of a word that describes the adult children of your sibling. I came up with the following: niephew, neceph, nie, nepto, nieto, and neech. I understand that such terms may be used to describe one's sibling child who's a youth. Could a world like 'neech' be designated for young children, while a word like 'nepto' be designated for adults? I am most welcome to other suggestions. Thanks.
the thing about creating a language, or even just words, is that you get to decide everything. if you decide that "neech" is designated for young children and "nepto" is designated for adults, then it is so, no questions asked.
i don't really have any suggestions as i'm not entirely sure where you're trying to go with these words. are they meant to be gender neutral/genderless alternatives for "niece" and "nephew" in English, or...?
the only real comments i have are that:
"neech" makes me think of "leech", which probably isn't a good thing.
"niephew" obviously comes from "nephew", but how would it be pronounced...?
a part of me (a part of me which i really hate) wants to automatically code/read "nepto" and "nieto" as masculine because they look similar to words found in certain romance languages where words have grammatical gender and -o denotes masculine gender.
would all of these words be inflected with the typical -s/-es for pluralization? you may want to take this into consideration too. "neeches" reminds me even more of "leeches" and "neptos" sounds like a Roman deity. :)
....and i'm sure that none of those comments really help you at all, sorry! but all the best with your words. :)