What is the singular and plural and plural possessive of son-in-law?

The possessive form of the singular noun son-in-law is son-in-law's.

Example: I'm making a cake for my son-in-law's birthday.

The plural form of the noun son-in-law is sons-in-law.
The plural possessive form is sons-in-law's.

Example: Both of my sons-in-law's birthdays are on the same day.