The 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.
Sometimes specifically a parent-in-law of one's child, for which the dedicated terms co-mother-in-law, co-father-in-law are rare.
A relative by marriage (or through affinity).