Definition from Wiktionary, the free dictionary
co-grandmother (plural co-grandmothers)
- 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”, Boston Globe, 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 02, Kate Tuttle, “A Grandmother's Right? Or Totally Obnoxious?”, 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)