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