1) In the natural and common sense (Matt. 1:2; Luke 3:1, 19).
2) A near relation, a cousin (Gen. 13:8; 14:16; Matt. 12:46; John 7:3; Acts 1:14; Gal. 1:19).
3) Simply a fellow-countryman (Matt. 5:47; Acts 3:22; Heb. 7:5).
4) A disciple or follower (Matt. 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; Matt. 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 Thess. 5:1). Brethren of Jesus (Matt. 1:25; 12:46, 50: Mark 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.
Easton's Bible Dictionary. M.G. Easton. 1897.