great-niece-in-law Share definition Flag 1. The granddaughter-in-law of one's sibling. 2. The granddaughter of one's spouse's sibling. My great-niece-in-law is a good person. great-niece-in-law by Elivaldeq November 24, 2019 0 0 Get merch Get the great-niece-in-law mug. Buy Now — $32.95 See more merch