Blood relatives, especially one’s siblings; also used of one’s close male associates. Cain and Abel are blood brothers (Gen 4:9), as are Jacob and Esau (Gen 25:26) and Simon and Andrew (Mark 1:16). “Brother” also embraces clans, compatriots, and allies. David calls Jonathan his “brother” (2Sam 1:26). Allied kings (Solomon and Hiram of Tyre) are “brothers” (1Kgs 9:13). Israelite priests constitute a brotherhood (2Chr 29:34). “Brothers” is also used to express the spiritual relationship of co-religionists. Jesus calls brothers those “who hear the word of God and do it” (Luke 8:21). Paul regularly speaks of his co-religionists as “brothers” (1Cor 1:10; 1Thess 1:4).
