grammar : Possessive Case of the Noun

The plural of mother-in-law is mothers-in-law. The possessive of mother-in-law is mother-in-law’s. To get the plural possessive, do both those things: put the plural S on mother and the possessive S and apostrophe on law: “It was their mothers-in-law’s idea.”

Tim and Stephanie’s apartment shares a single apostrophe and S because they share possession of the apartment. But they own their cars separately, so they’re Stephanie’s and Tim’s cars.