<P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

What do mother in laws call each other