nibling
Used especially as a gender-neutral term: the child of one's sibling or sibling-in-law; one's nephew or niece.
Aunts and uncles are concerned with the education of their niblings and may play a minor role in the ultimate arrangement of a marriage for the nibling.
Where it comes from
Blend of nephew or niece + sibling, coined by the American linguist Samuel Elmo Martin (1924–2009) in 1951.
Adapted from the Wiktionary entry nibling (CC BY-SA 4.0).