co-grandparent

• co- +‎ grandparent

co-grandparent (plural co-grandparents)

1. the parent of one's son- or daughter-in-law, as a common grandparent to one's children
   • 2009 May 19, Beth Teitell, “Horns Come Out When Grandparents Envy Each Other”, in Boston Globe‎, page B7:

     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.

• co-parents-in-law (relative to the children rather than to the grandchildren)

• co-grandmother
• co-grandfather

• co-grandfather-in-law
• co-grandmother-in-law
• co-in-law