The term "brother" can refer to:
A man or boy in relation to his siblings
A close male friend
A fellow member of the same race, creed, profession, or organization.
What Does "Brother" Mean in the Old Testament?
In the Old Testament, the Hebrew word for “brother” refers to male children who share at least one parent. Joseph and Benjamin were both children of Jacob and Rachel (Genesis 35:24). Still, Jacob's other sons were also Joseph’s brothers (Genesis 42:6).
The love Joseph had for Benjamin was special and is not always shared between brothers. Cain killed his brother Abel (Genesis 4:8). Esau hated his brother, Jacob (27:41). A brother can be a bad influence (Deuteronomy 13:6–7), but ideally he helps in times of need (Proverbs 17:17). The law of levirate marriage meant that if a man died leaving a childless widow, his brother had to marry her and have children to continue his brother's family name (Deuteronomy 25:5).
David called Jonathan his “brother” even though they were not related (2 Samuel 1:26). A fellow Israelite could be called brother. The relationship came with certain duties: no one could charge interest on their brother nor enslave him (Leviticus 25:35–43).
What Does "Brother" Mean in the New Testament?
In the New Testament, the Greek word describes natural brothers, like Andrew and Peter (John 1:41). Jesus had four brothers (Mark 6:3). (Roman Catholics believe they were Jesus's cousins, but the Greek does not use the word for cousin.) Jesus’s brothers did not believe in him at first (John 7:5). Still, after the resurrection, they joined the Christian community (Acts 1:14). Jesus taught that his disciples had one Father (God) and were all brothers (Matthew 23:8–9). He also was their brother (Matthew 28:10).
Early Christians called each other "brother" (Acts 9:17; Colossians 1:1). Twice, the Christian community is called “the brotherhood” (1 Peter 2:17; 5:9). Being a part of the Christian brotherhood comes with certain duties:
Controlling sexual desires (1 Thessalonians 4:6)
Providing material help when needed (James 2:15–16)
Avoiding cause for offence (Romans 14:13).
Christians should not “go to law” or sue their brothers (1 Corinthians 6:5–6)
They must resolve their problems either personally in the church (Matthew 18:15–17)
A Christian's relationship with their brother is significant because they cannot worship God if he is not in harmony with his brother (Matthew 5:23–24).