Definition from Wiktionary, the free dictionary
co-mother-in-law (plural co-mothers-in-law)
- (uncommon) The mother of one's son- or daughter-in-law; that is, the mother-in-law of one's son or daughter, or, the mother of one spouse in relation to the parents of the other spouse.
- (in the plural) The relationship between women whose children marry each other; the mother of the bride vis-à-vis the mother of the groom.
1901, Honoré de Balzac, (trans.) Francis MacNamara, The Physiology of Marriage, page 334:
- After dinner, the discerning eye of the co-mother-in-law divines the work of darkness. Your wife also is an expectant mother !
1912, Sarah Grand, Adnam's Orchard, page 508–509:
- 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.
1956, Margaret Mead, New lives for old: cultural transformation—Manus, 1928-1953, page 381:
- […] 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.
- In conversation, the generic "in-law" is generally used, with context left to disambiguate. Once grandchildren are born, the term co-grandmother may be used if the focus is on the relationship through the grandchildren rather than though the married couple.
mother of one's son- or daughter-in-law