A sister-in-law is a person related to someone by marriage rather than by blood. Commonly the term refers to the wife of a person's brother or sister, but it also applies to a spouse's sister and, in everyday use, to other marital connections such as the wife of a spouse's sibling. The word belongs to a wider class of "in-law" kinship terms that mark relationships created through marriage.

Definition and common types

Different English usages identify several distinct relationships as sisters-in-law. Typical categories include:

  • the spouse of one's sibling (for example, a brother's wife),
  • a sibling of one's spouse (for example, a wife's sister or a husband's sister),
  • the spouse of a spouse's sibling (for example, a spouse's brother's wife),
  • in some families, the partner of a sibling-in-law, depending on local usage.

Gender-neutral and same-sex marriages mean that "sister-in-law" can refer to a woman connected by any of these marital routes, while corresponding male relations are called brothers-in-law. The conventional plural form is "sisters-in-law."

Social role, etiquette and variation

The social expectations placed on sisters-in-law vary widely. In many families they are central participants in holidays, caregiving, and child-rearing networks; in others they remain more peripheral. Etiquette norms—about gift-giving, degrees of formality, or involvement in decision-making—are shaped by culture, religion and personal relationships rather than by the kinship term itself.

Legal rights and obligations for in-laws also differ by jurisdiction. Because the relation is by marriage, sisters-in-law are not blood relatives; however, they may acquire certain rights or responsibilities through family law, inheritance rules, or domestic arrangements, depending on local statutes and whether the marriage or partnership is recognized.

Practical distinctions matter in everyday life: for example, whether someone refers to another as "my sister-in-law" can signal closeness or a formal family tie. For more on marriage and kinship terminology, see related entries and resources: wife and general introductions to family law and kinship vocabulary.