A daughter-in-law is the wife of your son. Welcome a daughter-in-law.
The in-law part means related in law rather than by blood, which the phrase says plainly. A daughter-in-law relationship is the subject of more proverbs than almost any other.
1noundaughter-in-lawThe wife of your son.
“His daughter-in-law ran the family business for thirty years.”
daughter-in-law is a relative.
Daughter plus in-law, meaning related through canon law rather than by birth.
Middle English
doghter in lawe
English, today
daughter-in-law
From Middle English doghter in lawe; equivalent to daughter + -in-law.
Etymology from Wiktionary, CC BY-SA