Wiktionary
co-in-law
n. (context rare English) A distant in-law: A relationship by marriage with a separation by three degrees (two people), compared to prototypical in-laws, where the separation is two/one. The separation may be two degrees of blood kin and one of marriage (addressing the mother of the husband of one's daughter) or one degree blood and two of marriage (addressing the wife of the brother of one's husband).