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.
Co-relative-in-law: A relative of one's generation, separated by two degrees of marriage.
A relative of one's generation, separated by one degree of marriage.