The form "father's in law" is incorrect.
The possessive form of the singular, compound noun father in law is father in law's.
Example: We completed the project with my father in law's help.
The possessive form of the singular noun son-in-law is son-in-law's.Example: My son-in-law's name is Harry.
The possessive form of the singular noun attorney-at-law is attorney-at-law's.Example: This attorney-at-law's experience includes representing defendants in criminal cases.
The possessive form of the compound noun sister-in-law is sister-in-law's.Example: My sister-in-law's birthday is the same day as mine.
The possessive form of the plural noun fathers is: the fathers' rights.
The singular possessive of court is court's.
The plural possessive form is fathers-in-law's.
The singular possessive of father-in-law is father-in-law's. "This is my father-in-law's house."
The possessive form of the singular noun son-in-law is son-in-law's.Example: My son-in-law's name is Harry.
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.
The possessive form of the singular noun sister-in-law is sister-in-law's.Example: We're planning a party for my sister-in-law's birthday.
The possessive form of the singular noun attorney-at-law is attorney-at-law's.Example: This attorney-at-law's experience includes representing defendants in criminal cases.
The possessive form of the compound noun sister-in-law is sister-in-law's.Example: My sister-in-law's birthday is the same day as mine.
The possessive form of the singular noun mother-in-law is mother-in-law's.example: The party is for my mother-in-law's birthday.
The possessive form of the singular noun mother-in-law is mother-in-law's.example: The party is for my mother-in-law's birthday.
The possessive form of the singular noun mother-in-law is mother-in-law's.example: The party is for my mother-in-law's birthday.
The possessive form of the singular noun son-in-law is son-in-law's.Example: I finished painting the house with my son-in-law's help.
The possessive form of the singular noun father-in-law is father-in-law's.Example: We're spending the weekend at my father-in-law's cabin.