The Greek noun adelphē (ἀδελφή) means sister — both in the biological sense (a woman who shares the same parents) and in the spiritual sense (a fellow member of the body of Christ, a co-heir of grace). It appears approximately 26 times in the New Testament. Jesus uses it in His teaching on the true family of God (Mark 3:35); Mary and Martha are named as adelphē (sisters) in John 11; and Paul addresses women in the church as adelphē (sisters) in Romans 16 and elsewhere.
The term adelphē in the New Testament is a revolution in social relationship. In the Greco-Roman world, religious communities were hierarchical structures of patrons and clients. But in Christ, the church becomes a family — brothers and brothers, brothers and sisters bound not by blood but by the Spirit. When Paul writes "treat younger women as sisters, with absolute purity" (1 Timothy 5:2), he is reordering all male-female dynamics through the lens of adelphē. Calling a woman "sister" changes how you treat her — not as an object, threat, or subordinate, but as family. Jesus' declaration in Mark 3:35 — "Whoever does God's will is my brother and sister and mother" — makes adelphē the highest possible relational category: membership in the family of God, with Jesus Himself as brother (Hebrews 2:11).