mocking and quoting bigots I've seen so far
:liberalism: turns to 
"you know it's hexbear when the tankie pronoun parade shows up to force pictures of pigshit in front of you!"* real quick
[*this is paraphrased, I can't remember the exact original quote]
Or "so suddenly I'm Hitler because i think people calling themselves Fae gender is dumb"
Or "why should i need to learn a new set of different pronouns for every person I meet?" (this followed two comments after "I'm literally not a transphobe, I work with plenty of trans colleagues who feel safe around me" (average lib being scratched in real time by having to acknowledge neopronouns))
.
All of these shitheads claimed to be allies when they started interacting but our brilliant pronoun tags immediately made them out themselves as the bigots they are who only pretend to be allies because it looks bad to be openly and proudly transphobic
TL;DR: 
To be clear, from what I understand, "niblings" is gender neutral for niece/nephew, not brother/sister (which is just the regular word "siblings" you already know). There could be some disagreement about that. But I call the young person with DID of various genders who I have adopted as e-family my "niblings". I knew the term before because I just wanted to know what the gender neutral for that was.
Huh. Interesting. I had found it as a similar tier of relation to gender terms for sibling. But now that you mention it I think I recall that usage from before. This has made sense as it seems a mutation of sibling + nb, in a similar manner to enby.
The term "sibling" itself doesn't seem quite specific enough to me in such usage as while it is gender-neutral, it does so by being extremely general and passive, while my intent would be to affirmatively communicate those of a sibling-tier relation who identify as non-binary genders not linguistically accounted for or acknowledged in recent history.