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 singular possessive form of "father's in law" is "father-in-law's."
The singular possessive form of "son-in-law" is "son-in-law's."
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 singular possessive of "sister-in-law" is "sister-in-law's."
The possessive noun phrase for the right of fathers is "fathers' rights."
The possessive noun phrase is the fathers' rights.
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 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.
The singular possessive form of "son-in-law" is "son-in-law's."
The singular possessive form of "sister-in-law" is "sister-in-law's."
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 singular possessive of "sister-in-law" is "sister-in-law's."
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 brother-in-law is brother-in-law's.example: I'm making a cake for my brother-in-law's birthday.