Dictionary of Biblical Words:
Used over 200 times as a common title amongst the children of God, and given by Christ in Matt. 23:8. “One is your master, even Christ, and all ye are brethren.” It is a title that is the birthright of every member of God’s family. The “many sons” whom He brings to glory, and whom the Lord is not ashamed to call His “brethren.” To apply the name in any narrower sense is contrary to the teaching of Scripture.
Concise Bible Dictionary:
Besides the literal meaning of the term, it was used for persons morally or nationally associated together; also for those who formed the Jewish nation (Acts 2:29,37 where the expression “Men and brethren” should be translated simply “Brethren”; Rom. 9:3). It was applied by the Lord to His disciples: “Go to my brethren, and say unto them, I ascend to my Father, and your Father; and to my God, and your God” (John 20:17). “He is not ashamed to call them brethren” (Heb. 2:11; Psa. 22:22). The Lord has placed the Christian in His own relationship as man with His Father and God, and “He that sanctifieth and they who are sanctified are all of one.” In accordance with this the saints are constantly addressed in the Epistles as “brethren,” and referred to in the Acts as “the brethren.”