brother: a male who shares your bond, from a born sibling out to a fellow member of God's family
At its core this word names a male sibling, someone born of the same parents. But the New Testament writers stretch it outward in widening circles, and the same word does all the work. From a shared mother and father it reaches to a shared ancestry (a fellow Jew is a "brother"), then to a shared faith (a fellow Christian is a "brother"), then to the simple fact of standing in relationship with another person at all (the "brother" you must not be angry with, or must forgive).
The move that matters most is the third one. By far the most common use in the New Testament is not the family one but the church one: believers call each other "brothers" because they understand themselves to be children of one Father, made kin by faith rather than by blood. This is why the address "brothers" opens so many letters and so many speeches.
The senses are not separate words but one relationship pictured at different distances. Whether the bond is birth, nation, faith, or shared humanity, the claim is the same: this person is not a stranger to you, and that fact carries obligations of loyalty, care, and forbearance.
The masculine form is also used inclusively for a mixed group of believers, men and women together, which is why many modern translations render the plural "brothers and sisters."