No, the form son-in-laws' is an incorrect form.
The plural form of the compound 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.
The plural possessive form of "son-in-law" is "sons-in-law's".
The plural form is echoes. The plural possessive is echoes'.
The plural form is Mexicans. The plural possessive is Mexicans'.
The plural is lads. The plural possessive is lads'.
The plural is relatives. The plural possessive is relatives'.
The correct form is "its" for the possessive form in the plural. "Its" is used for both the singular and plural possessive forms, without an apostrophe.
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.
The correct form is "its" for the possessive form in the plural. "Its" is used for both the singular and plural possessive forms, without an apostrophe.
The plural is briefs. The plural possessive is briefs'.
The plural is lads. The plural possessive is lads'.
The plural is trollies. The plural possessive is trollies'.
The plural is stations. The plural possessive is stations'.
The plural is relatives. The plural possessive is relatives'.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive form is heart's; the plural possessive form is hearts'.
The plural form is homes; the plural possessive is homes'.
The plural of rose is roses. The plural possessive is roses'.
The singular possessive is biker's; the plural possessive is bikers'.