What is the plural possessive of the word father-in-law?

Answer 1

The plural form of the noun father-in-law is fathers-in-law.

The plural possessive form is fathers-in-law's.

example: The fathers-in-law's project was a gift for our anniversary.

Answer 2

The plural possessive of "father-in-law" is "fathers-in-law's" if you are referring to the possession of multiple fathers-in-law collectively. Each father-in-law would individually be "father-in-law's" in the possessive form.