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A person's mother-in-law is the mother of his or her spouse. Mothers-in-law are often stereotyped in mother-in-law jokes.
A person is a son-in-law or daughter-in-law to the mother-in-law.
She and the father-in-law are the parents of the spouse and in a nuclear family of his or her sisters-in-law and brothers-in-law. They all together are the in-laws.