A mother-in-law is the mother of someone's spouse. When two people get married, each person's parents become the other person's in-laws. So if your older sister marries someone, her husband's mother becomes her mother-in-law, and your own mother becomes his mother-in-law.

The relationship can feel unusual at first because you're suddenly connected to someone through marriage rather than being born into the same family. Your mother-in-law isn't someone you grew up with, yet she's now part of your extended family. She raised the person you married, which means she probably shaped many of their habits, values, and personality traits.

Throughout history, mothers-in-law have gotten a complicated reputation. But, like any relationship between adults, the relationship depends on the individuals involved and how much effort they put into understanding and respecting each other.

The term uses “in-law” because these relationships are created by law through the legal bond of marriage. Other in-law relationships include father-in-law (your spouse's father), brother-in-law (your spouse's brother or your sibling's husband), and sister-in-law (your spouse's sister or your sibling's wife or spouse).