An in-law is a relative you gain through marriage rather than birth. When your sister marries someone, that person becomes your brother-in-law. When you marry someone, their parents become your mother-in-law and father-in-law. Your spouse's siblings are your brothers-in-law and sisters-in-law.

The term recognizes that marriage creates new family connections. These relationships matter because they expand your family circle. Your in-laws aren't blood relatives, but they're still family in the eyes of the law and tradition.

In-law relationships can be wonderful, complicated, or anything in between, just like relationships with biological family. Some people become incredibly close with their in-laws, treating them exactly like their own parents or siblings. Others maintain more distant but respectful relationships.

The word appears in many common phrases: people talk about visiting their in-laws for the holidays, or mention that their mother-in-law taught them to cook a special recipe. These relationships have inspired countless stories and jokes throughout history, but at their heart, in-laws are simply the relatives you gain when you marry someone.