co-grandmother

• co- +‎ grandmother

co-grandmother (plural co-grandmothers)

1. the mother of one's son- or daughter-in-law, as common grandmother of their children; the relationship between women who have grandchildren in common; maternal grandmother vis-à-vis paternal grandmother.
   • 2009 May 19, Beth Teitell, “Horns Come Out When Grandparents Envy Each Other”, in Boston Globe‎, archived from the original on 3 June 2009, page B7:

     Talk to enough grandparents and one thing becomes clear: Because she's closer to the mother of the children, the maternal grandmother generally has a slight advantage. Or, as Audrey Berson […] put it: "She's my daughter, so I have the edge." Even so, Berson's co-grandmother works in the same town as the grandkids, and she sometimes meets the bus after school. "It bothers me," Berson said. "I wish it were me."

   • 2008 December 2, Kate Tuttle, “A Grandmother's Right? Or Totally Obnoxious?”, in Strollerderby‎:

     A Canadian grandmother's essay about essentially crashing her daughter-in-law's birth […] Rhona Bennett writes of […] hopping a train to Montreal, and (in cahoots with her co-grandmother) bum-rushing the hospital […] The two bubbies called the hospital […] and naturally were rebuffed, so off they went, in search of information, affirmation, and a grandbaby.

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

• co-grandfather
• co-grandparents

• co-grandmother-in-law
• co-in-law
• co-aunt