Expressing blood brothers or sisters: this is the most basic meaning, used to express male or female kinship with parents.
Express the relationship between friends, friends or companions: Sometimes people will call their friends, friends or companions "brothers", indicating close friendship and deep friendship.
Expressing peer, peer or homosexual relationship: In some social groups, people will also use "brother" to express peer, peer or homosexual relationship, such as gangs, the army, the police and so on.
Brothers or sisters: Sometimes people also refer to brothers or sisters as "brothers" or "sisters".
In short, the meaning of the word "brother" is varied, and the specific meaning depends on the usage scene and context.