A male having the same parent or parents as another; in Hebrew, ʼach, and in Greek, a·del·phosʹ. Full brothers mentioned in the Bible, sons of the same father and the same mother, include Cain and Abel, sons of Adam and Eve  (Ge 4:1, 2; 1Jo 3:12); Jacob and Esau, twin sons of Isaac and Rebekah (Ge 25:24-26); James and John, sons of Zebedee and his wife (Mt 4:21; 27:56; compare Jg 8:19). Moses and Aaron were brothers of Miriam (Nu 26:59); Lazarus was brother to Martha and Mary. (Joh 11:1, 19) “Brothers” also designates half brothers, those with the same father but a different mother, as in the case of Jacob’s 12 sons by four different women (Ge 35:22-26; 37:4; 42:3, 4, 13); also, offspring of the same mother but of different fathers, as in the case of Jesus and his brothers, and possibly in that of David’s relationship to his sisters.​—Mt 13:55; 1Ch 2:13-16; 2Sa 17:25; see “Brothers of Jesus” below.

The term “brother,” however, was not limited to the immediate fleshly relationship. Abraham and Laban referred to their nephews Lot and Jacob respectively as brothers. (Ge 11:27; 13:8; 14:14, 16; 29:10, 12, 15; compare Le 10:4.) Fellow members of the same tribe in Israel enjoyed a brotherly relationship (2Sa 19:12, 13; Nu 8:26), and in a still larger sense the entire nation of Israel were brothers, offspring, as they were, of one common father Jacob, and they were united in worship of the same God, Jehovah. (Ex 2:11; De 15:12; Mt 5:47; Ac 3:17, 22; 7:23; Ro 9:3) Even the Edomites, who descended from Abraham through Jacob’s twin brother Esau, thereby being related to Israel, were called brothers. (Nu 20:14) The reunited kingdoms of Judah and Israel were referred to as in a “brotherhood” (Heb., ʼa·chawahʹ).​—Zec 11:14.

“Brother” is also applied to those united in a general cause and having similar aims and purposes. For example, King Hiram of Tyre called King Solomon his brother, not simply because he was an equal in rank and position but also perhaps because of mutual interests in supplying timbers and other things for the temple. (1Ki 9:13; 5:1-12) “Look! How good and how pleasant it is for brothers to dwell together in unity!” David wrote, implying that it is not blood relations alone that make for peace and unity between fleshly brothers. (Ps 133:1) In fact, mutual affection and interest, not common parentage, prompted David to call Jonathan his brother. (2Sa 1:26) Companions having similar natures and dispositions, even when such are bad, are properly called brothers.​—Pr 18:9.

In the patriarchal society and under the Mosaic Law, certain privileges and obligations were assumed by fleshly brothers. With the death of the father, the oldest brother, the firstborn, received a double share of the family inheritance and the responsibility of acting as head for the family. A fleshly brother was first in line for the right of repurchase, levirate marriage, and avenging blood. (Le 25:48, 49; De 25:5) Incestuous relations between brother and sister were strictly forbidden by the Mosaic Law.​—Le 18:9; De 27:22.

In the Christian congregation members enjoy a common spiritual relationship analogous to that of brothers. Jesus called his disciples brothers. (Mt 25:40; 28:10; Joh 20:17) He strongly emphasized this relationship, saying: “Whoever does the will of my Father . . . , the same is my brother, and sister, and mother.” (Mt 12:48-50) Hence blood relatives must be loved less than Christ and left behind on his account if necessary. (Mt 10:37; 19:29; Lu 14:26) Indeed, brother may deliver brother over to death. (Mr 13:12) The term “brother” extends out beyond the immediate associates of Jesus to include the whole congregation of believers (Mt 23:8; Heb 2:17), “the whole association of brothers” “who have the work of witnessing to Jesus.” (1Pe 2:17; 5:9; Re 19:10) Such an association of spiritual brothers shows “brotherly love” in its fullest measure.​—Ro 12:10; Heb 13:1.

Peter at Pentecost addressed those from faraway lands, including proselytes, all as “brothers.” (Ac 2:8-10, 29, 37) Sometimes male Christian believers were distinguished as “brothers” and females as “sisters” (1Co 7:14, 15), but generally “brothers” was the accepted greeting to mixed groups and was not restricted to males. (Ac 1:15; Ro 1:13; 1Th 1:4) The term is used in this sense in all but three of the inspired Christian letters (Titus, 2 John, Jude) and in the works of early church writers. The apostles warned against “false brothers” who infiltrated the congregations.​—2Co 11:26; Ga 2:4.

Brothers of Jesus. The four Gospels, the Acts of Apostles, and two of Paul’s letters mention “the Lord’s brothers,” “the brother of the Lord,” “his brothers,” “his sisters,” naming four of the “brothers”: James, Joseph, Simon, and Judas. (Mt 12:46; 13:55, 56; Mr 3:31; Lu 8:19; Joh 2:12; Ac 1:14; 1Co 9:5; Ga 1:19) The majority of Bible scholars accept the cumulative evidence that Jesus had at least four brothers and two sisters and that all were offspring of Joseph and Mary by natural means after the miraculous birth of Jesus.

The arbitrary notions that these brothers of Jesus were sons of Joseph by a former marriage, or by levirate marriage with Joseph’s sister-in-law, must be classified as fictitious, since there is no factual confirmation or even a suggestion to this effect in the Scriptures. The claim that “brother” (a·del·phosʹ) here means “cousin” (a·ne·psi·osʹ) is a theoretical contention, the invention of which is  credited to Jerome, and dates back no earlier than 383 C.E. Not only does Jerome fail to cite any support for his newborn hypothesis but in later writings he wavers in his opinions and even expresses misgivings about his “cousin theory.” J. B. Lightfoot states that “St Jerome pleaded no traditional authority for his theory, and that therefore the evidence in its favour is to be sought in Scripture alone. I have examined the scriptural evidence, and the . . . combination of difficulties . . . more than counterbalances these secondary arguments in its favour, and in fact must lead to its rejection.”​—St. Paul’s Epistle to the Galatians, London, 1874, p. 258.

In the Greek Scriptures where the account involved a nephew or cousin, a·del·phosʹ is not used. Rather, the relationship is explained, as “the son of Paul’s sister” or “Mark the cousin [a·ne·psi·osʹ] of Barnabas.” (Ac 23:16; Col 4:10) In Luke 21:16, the Greek words syg·ge·nonʹ (relatives, such as cousins) and a·del·phonʹ (brothers) both occur, showing that the terms are not used loosely or indiscriminately in the Greek Scriptures.

When, during Jesus’ ministry, “his brothers were, in fact, not exercising faith in him,” this would certainly rule out their being his brothers in a spiritual sense. (Joh 7:3-5) Jesus contrasted these fleshly brothers with his disciples, who believed in him and who were his spiritual brothers. (Mt 12:46-50; Mr 3:31-35; Lu 8:19-21) This lack of faith on the part of his fleshly brothers prohibits identifying them with apostles of the same names: James, Simon, Judas; they are explicitly distinguished from Jesus’ disciples.​—Joh 2:12.

The relationship these fleshly brothers of Jesus had with his mother Mary also indicates they were her children rather than more distant relatives. They are usually mentioned in association with her. Statements to the effect that Jesus was Mary’s “firstborn” (Lu 2:7), and that Joseph “had no intercourse with her until she gave birth to a son,” also support the view that Joseph and Mary had other children. (Mt 1:25) Even Nazarene neighbors recognized and identified Jesus as “the brother of James and Joseph and Judas and Simon,” adding, “And his sisters are here with us, are they not?”​—Mr 6:3.

In the light of these scriptures, the question is asked: Why, then, should Jesus just before his death entrust the care of his mother Mary to the apostle John instead of to his fleshly brothers? (Joh 19:26, 27) Manifestly because Jesus’ cousin, the apostle John, was a man who had proved his faith, he was the disciple whom Jesus loved so dearly, and this spiritual relationship transcended that of the flesh; in fact, there is no indication that his fleshly brothers were, as yet, disciples of Jesus.

After Jesus’ resurrection his fleshly brothers changed their doubting attitude, for they were present with their mother and the apostles when assembled for prayer after Jesus’ ascension. (Ac 1:14) This suggests that they were present also at the outpouring of the holy spirit on the day of Pentecost. Jesus’ brother James, who was singled out prominently among the older men of the governing body in Jerusalem, wrote the letter bearing his name. (Ac 12:17; 15:13; 21:18; Ga 1:19; Jas 1:1) Jesus’ brother Jude penned the book bearing his name. (Jude 1, 17) Paul indicates that at least some of Jesus’ brothers were married.​—1Co 9:5.