Mate's kin (2-4)
(Other definitions for in-law that I've seen before include "Relative by marriage" , "Relation by marriage" , "Spouse's relation" , "Family member, by marriage" , "Near relative" .)
(Other definitions for in-laws that I've seen before include "Relations by marriage" , "A person's relatives by marriage" , "Relatives by marriage 2-4)" , "Christmas is when you might bid them welcome" , "Spouse's relations" .)