The plural form is fathers-in-law, because you're talking about more than one person. The rest of the compound, -in-law, remains singular because they share a title.
The plural form of "father-in-law" is "fathers-in-law."
The plural form for the demonstrative pronoun this is these.
The plural form of the word "ranch" is "ranches."
The plural form for the word 'pioneer' is pioneers.
The plural form of the word "pharynx" is "pharynges."
The plural form of vow is vows.
There is no plural form for the word, countries. This word itself is a plural.
The plural form of the word "inferno" is "infernos."
There is no plural word for if.
The plural form is data.
In Chinese, the plural form of the word "minute" is "分钟" (fēnzhōng).
The plural of diary is diaries.
The plural form for the word 'pioneer' is pioneers.
The plural form of the word "baby" is "babies."
The plural form of the word 'deed' is 'deeds'.
The plural form of the word signal is signals.
The plural form of the word gelato is gelati.
The plural form of the word "search" is "searches".