co-grandparent

plural co-grandparents

1. The parent of one's son- or daughter-in-law as a common grandparent to children of that union.

   1 more example

   Most grandparents aren't engaged in a duel with their co-grandparents, but the desire to be the most beloved, most generous grandparent often simmers beneath the surface. […] “It can get to the point where they can't speak to the co-grandparents, or the parents can't have events where they're both there,” she said.

   2009 May 19, Beth Teitell, “Horns Come Out When Grandparents Envy Each Other”, in Boston Globe, archived from the original on 03 Jun 2009, page B7

   Synonyms: co-parent-in-law