Some twenty-one years ago, my sister gave birth to fraternal twins, Ian and Megan. In last year’s Christmas newsletter, the family announced that Megan had come out as non-binary, and had adopted the name Min. Min’s pronouns are “they,” “them,” and “their.”
It just now hit me that I am still Min’s uncle, but I don’t know if Min still thinks of themself as my niece. Is there a standard name for what a non-binary person is to an uncle or aunt?
I’ll be asking m my sister in the morning, but any knowledgeable response before then would be appreciated.
The simple answer is to ask them. They may not even have thought about it yet. A gender neutral term for niece/nephew is, apparently, “nibling”.
Thanks for that. I, too, need a gender neutral term for niece/nephew. I was toying with “neicfew”.
kaylasdad - good for you. It’s pretty critical that relatives are gender affirming. Or to say a different way, gender affirming relatives go a long way. Good on ya.
I hadn’t heard niblings although my eldest child is gender fluid non-binary.
Pronouns are tough but get easier after you say them out loud a few hundred times. 
I had the same issue for several years, and never really arrived at a good noun to use. When she was about 10 years old, the daughter of some dear friends of mine “adopted” me as her uncle, and declared herself to be my niece (she has several actual uncles, all of whom live in Europe, and she barely knows them).
Several years later, the adolescent “she” became “they,” when they decided that they were gender-fluid. They still thought of me as their uncle, but they and I never did settle on an appropriate non-binary noun to replace “niece,” so I would just refer to them as “my [their name],” which they liked to hear. 
(Around age 19, they decided that they identified as female, and went back to “she/her.”)
I call mine, collectively, nephlets. But maybe I’ll try out niblings.
zeroth cousin once removed?
What’s the gender neutral term of aunt/uncle?
I googled it and found “pibling.”
Sheri Tepper used the term “thalan” in her True Game books to denote any child of any of your siblings, gender neutral all the way. A word like that would definitely be useful.
Even with cisgender binary relatives, such a word would still be useful in the plural. My sister has three kids, and so far as I know there’s nothing non-mainstream or uncommon about any of their gender identities, but there’s no one word to refer to all three of them.
Yes, that’s why I use “nephlets”. Just a made-up word, and not the best, since it sounds a lot more like nephew than like niece.
I guess collectively I also say “the cousins”. they are technically 0th cousins once removed.
I like “nibling” very much and may start using it though none of my niblings are genderfluid (at least not that I know of).
There are creatures called niblings in Tove jansson’s Moomin books. I believe they are small animals who bite off people’s noses.
What I would like is a word to describe the offspring of my first cousins, of whom there are many. I can certainly use “cousins,” though it’s a little vague; my oldest first cousin is 58, my youngest first cousin once removed is 1. “First cousins once removed” is more accurate but bulky. I tried “FCOR” at one point (pronounced F-corps) but it didn’t really get off the ground. One of my FCOR calls me “second uncle” based on me being the father of his second cousins, so I refer to him as “second nephew.” I suppose “second niblings” would work but boy we are down in the weeds by now.
“Second niblings” sounds like a hobbit word for a snack.
I got a piece of jewelry for my nibling. I think I’ll write a story about it and call it “The Ring of the Nibling”.
Some people find being right in the middle of language creation to be frustrating, but I think it is exciting and fun.
You could shorten it to “s’uncle” and “s’niblings.”
Just chiming in to say you’re not alone here. When one of my sister’s kids came out as trans about 8 years ago, I spent most of the night tossing and turning over this very question. Had to Google at 3AM just to get to sleep!
I came up with “nibling” too, btw. It just makes sense when you consider sister-brother-sibling, nephew-niece-nibling.
As others have said, it takes some time to get your head straight, but wanting to get it right is the important first step. One odd thing I noticed about myself, when I was thinking about them, I had no problem with a new name and new pronouns, but when I went to talk, the old ones would come out. It’s like a different part of my brain was in charge of speaking. But even that got better with time.
I’ll go along with niblings and piblings if that’s the consensus that develops. But I’ll say right now that these sound like minor D&D races.