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.
If nibling has for most of its history merely puttered along, it is now moving with new purpose. While initially considered useful for its efficiency, it is now increasingly called upon as a means to gender accuracy:
My "nibling," the gender-neutral term for nephew or niece, is neither male nor female, but both.
— "Proud Aunt," "Ask Amy" in The Chicago Tribune, 9 Jan. 2014
There was that benevolent look for his genderqueer "nibling" (read: gender-neutral niece or nephew). It was big-eyed and full of wonder with a smile that gave you warmth.
— Joey Hood, The Nashville (Tennessee) Scene, 23 June 2016
Another [fan] suggested the term "nibling" to the singer, which is widely considered a gender-neutral term for a sibling's child.
— Chantal Da Silva, The Independent (UK), 24 July 2017