A sister-in-law is a woman who becomes part of a family through marriage, not through birth. She might be your brother's wife, your spouse's sister, or your spouse's brother's wife.

The relationship works both ways: if your brother marries someone, she becomes your sister-in-law, and you become hers. The same applies when you get married. Your husband or wife's sister becomes your sister-in-law, and you become her brother-in-law or sister-in-law.

At holidays, weddings, and family reunions, your sisters-in-law are there because their marriage makes them part of your family.