Technically, in English, “in-law” means any relative through marriage. http://www.merriam-webster.com/dictionary/in+laws?
It usually used for one’s child-in-law’s parents, but its more general definition is as in the film title “The In Laws” where it refers to the relationship between the two sets of parents (they are technically in laws to each other.)
In Hebrew/Yiddish the term “mechutanim” is used the same way — usual usage has it meaning one’s child-in-laws parents but more generally it means one’s relatives through marriage. (“Alle mechutanim – Mazel Tov!” refers to all relatives through marriage.)