Son in law is the person who is married to your daughter. An example of son in law is your daughter's husband. The husband of one's child.

Is it correct to say son in-laws?

There is only one law that makes him your daughter's husband. If you were referring to something belonging to your son-in law, you would say my son-in law's to show ownership. Therefore, sons-in law is correct.

What is the correct way to write son in law?

noun. \ ˈsən-ən-ˌlȯ \ plural sons-in-law.