A more thorough explanation:

DAUGHTER-IN-LAW

A daughter-in-law is the woman who is married to one's son.

• My son's wife is my daughter-in-law.
• When my daughter-in-law comes to visit, we always have a great time together.

The examples illustrate that a daughter-in-law is the wife of one's son. In the first example, the speaker refers to their son's wife as their daughter-in-law. In the second example, the speaker mentions spending time with their daughter-in-law, indicating that she is married to their son.