In the Scriptures the term is applied to full sisters and to half sisters, those having the same father but different mothers (Ge 34:1, 27; 1Ch 3:1-9), or the same mother but different fathers, as in the case of the sisters of Jesus.​—Mt 13:55, 56; Mr 6:3.

Some of Adam’s sons obviously married their sisters, since all humankind sprang from Adam and Eve. (Ge 3:20; 5:4) Adam’s wife Eve, as ‘bone of his bones and flesh of his flesh,’ was even more closely related than a sister. (Ge 2:22-24) There was no stigma attached to marriage to sisters or half sisters. The account reports that more than 2,000 years later, Abraham married Sarah his half sister. (Ge 20:2, 12) The Mosaic Law, some 430 years later, however, forbade such unions as incestuous. (Le 18:9, 11; 20:17) Doubtless, as the human race deteriorated from Adam’s original perfection, it became detrimental for closely related persons to marry.

“Sister” in its broader usage included fellow countrywomen of a nation. (Nu 25:17, 18) Nations or cities that had a close relationship or that carried on similar moral practices were likened to sisters.​—Jer 3:7-10; Eze 16:46, 48, 49, 55; 23:32, 33.

The Hebrew word for sister (ʼa·chohthʹ) is translated “the other” when describing the placing of objects in relationship to corresponding pieces in the tabernacle and in Ezekiel’s visions.​—Ex 26:3, 5, 6, 17; Eze 1:9, 23; 3:13.

In the Christian Congregation. Jesus taught that spiritual relationships take priority over fleshly ones. Those women who did his Father’s will were ‘sisters’ held in higher regard than mere fleshly relations. (Mt 12:50; Mr 3:34, 35) One willing to sever earthly ties, if necessary, for the sake of the Kingdom, will have a “hundredfold” of “sisters” and other ‘family’ relations now, plus “everlasting life” in the future. (Mt 19:29; Mr 10:29, 30; Lu 14:26) Women in the Christian congregation are called sisters, in a spiritual sense.​—Ro 16:1; 1Co 7:15; 9:5; Jas 2:15.

Figurative Use. Closeness to wisdom is encouraged by the wise writer Solomon when he stresses the importance of Jehovah’s commandments. “Say to wisdom: ‘You are my sister’; and may you call understanding itself ‘Kinswoman.’”​—Pr 7:4.