sis·ter / ˈsistər/ • n. 1. a woman or girl in relation to other daughters and sons of her parents. ∎ a half-sister, stepsister, or foster sister. ∎ a sister-in-law. ∎ a close female friend or associate, esp. a female fellow member of a labor union or other organization. ∎ (often Sister) a member of a religious order or congregation of women. ∎ a fellow woman seen in relation to feminist issues. ∎ inf. a black woman (chiefly used as a term of address by other black people). ∎ [usu. as adj.] a thing, esp. an organization, that bears a relationship to another of common origin or allegiance or mutual association: Eastern's sister airline, Continental a sister ship.2. (often Sister) Brit. a senior female nurse, typically in charge of a ward.DERIVATIVES: sis·ter·li·ness n.sis·ter·ly adj.ORIGIN: Old English, of Germanic origin; related to Dutch zuster and German Schwester, from an Indo-European root shared by Latin soror.