co-mother-in-law

plural co-mothers-in-law

1. uncommonThe mother of one's children-in-law; that is, the mother-in-law of one's children, or, the mother of one spouse in relation to the parents of the other spouse; either of two or more women whose children marry each other, such as the mother of a bride vis-à-vis the mother of the groom.

   She had a right of entrée to the Castle now as prospective co-mother-in-law with the duchess, and made the most of it.

   1912, Sarah Grand, Adnam's Orchard, pages 508–509

   2 more examples

   After dinner, the discerning eye of the co-mother-in-law divines the work of darkness. Your wife also is an expectant mother !

   1901, Honoré de Balzac, (trans.) Francis MacNamara, The Physiology of Marriage, page 334

   […] responded with temper and irritability toward her step-daughter, Anna, who was so demure and so complete a member of the new society, and toward Ngaoli, her co-mother-in-law, gaunt, old, married under the old system, a devoted church member, righteous mother of five children, who fortunately had no complications in her life.

   1956, Margaret Mead, New lives for old: cultural transformation—Manus, 1928-1953, page 381

   Synonyms: co-grandmother