The term "nibling" is used to refer to what?

Nibling is a term coined by linguist Samuel E. Martin in 1951 from nephew/niece by analogy with sibling. In addition to his scholarly linguistic works, Martin was interested in the teaching of East Asian languages, and he wrote a number of elementary texts and dictionaries for beginners. In 1951, in collaboration with Dr. Sane, Martin coined the term "nibling" as a gender-neutral word for a nephew or niece. The full definition of the word "nibling" would be expressed as "a child of one's sibling or of one's sibling-in-law (in other words, one's niece or nephew), especially in the plural or as a gender-neutral term".

It's popularity has never been actually high mainly because it can be confused with the present participle "nibbling" (from the verb to nibble), meaning "to take small bites out of" or "peck at" (as in he sat there, nibbling a cookie).