Easton's Bible Dictionary: M. G. Easton, M.A., D.D.

Brother

1. In the natural and common sense (Mt. 1:2; Lk. 3:1, 19).
2. A near relation, a cousin (Gen. 13:8, 14:16; Mt. 12:46; Jn. 7:3; Acts 1:14; Gal. 1:19).
3. Simply a fellow-countryman (Mt. 5:47; Acts 3:22; Heb. 7:5).
4. A disciple or follower (Mt. 25:40; Heb. 2:11, 12).
5. One of the same faith (Amos 1:9; Acts 9:30, 11:29; 1 Cor. 5:11); whence the early disciples of our Lord were known to each other as brethren.
6. A colleague in office (Ezra 3:2; 1 Cor. 1:1; 2 Cor. 1:1).
7. A fellow-man (Gen. 9:5, 19:7; Mt. 5:22, 23,24, 7:5; Heb. 2:17).
8. One beloved or closely united with another in affection (2 Sam. 1:26; Acts 6:3; 1 Th. 5:1). Brethren of Jesus (Mt. 1:25, 12:46, 50; Mk. 3:31, 32; Gal. 1:19; 1 Cor. 9:5, etc.) were probably the younger children of Joseph and Mary. Some have supposed that they may have been the children of Joseph by a former marriage, and others that they were the children of Mary, the Virgin's sister, and wife of Cleophas. The first interpretation, however, is the most natural.