A parent-in-law is a person who has a legal affinity with another by being the parent of the other's spouse. Many cultures and legal systems impose duties and responsibilities on persons connected by this relationship. A person is a son-in-law or daughter-in-law to the parents of the spouse, who are in turn also the parents of those sisters-in-law and brothers-in-law (if any) who are siblings of the spouse (as opposed to spouses of siblings). Together the members of this family affinity group are called the in-laws.
A father-in-law is the father of a person's spouse. Two men who are fathers-in-law to each other's children may be called co-fathers-in-law, or, if there are grandchildren, co-grandfathers.
A mother-in-law is the mother of a person's spouse. Two women who are mothers-in-law to each other's children may be called co-mothers-in-law, or, if there are grandchildren, co-grandmothers.
Parent-in-law Facts for Kids. Kiddle Encyclopedia.