Your in-laws are the family you gain through your husband or wife. Visit the in-laws, my in-laws, trouble with the in-laws.
The law in it is church law about marriage, which is why a mother-in-law is a mother in the eyes of the law rather than by blood. The plural covers the whole family at once, which the singular forms cannot do.
1nounin-lawsThe family you gain through your husband or wife.
“They spend every second Christmas with her in-laws.”
in-laws is family.
Also accepted in games: in laws
Practised in Family & People