Nibling is a gender-neutral term used to refer to a child of one’s sibling as a replacement for “niece” or “nephew”. The word is thought to have been coined in the early 1950s, but was relatively obscure for several decades before being revived in recent years.

https://www.merriam-webster.com/wordplay/words-were-watching-nibling

Huh. TIL. Thanks.