a brother: a male tied to you by birth, and by extension anyone bound to you by blood, nation, or loyal kinship
At its core this word names a male sibling, a son of the same parents. But Hebrew stretches the bond of brotherhood outward in widening circles, and the same word follows it the whole way. A nephew, an uncle, or a more distant blood relative can be called a brother. So can a fellow member of the same tribe or nation: in the laws of Israel "your brother" usually means a fellow Israelite, the neighbor you are bound to care for. Further out still, the word covers a friend or a treaty partner, someone joined to you not by blood but by a bond of loyalty as strong as family.
What holds these senses together is the idea of belonging to the same group and owing one another the duties that go with it. The closer the tie, the heavier the duty, which is why the word carries such moral weight: to wrong a brother, in any of these circles, is to betray a bond you were obligated to keep. Cain's question "Am I my brother's keeper?" hangs over the whole range.
Hebrew also uses the word in a fixed phrase, often "a man to his brother," to mean simply "one another." Here brother has thinned almost to grammar: it marks two things facing or matching each other, even two wings of a carved figure, without claiming any real kinship between them.
The sense is almost always settled by who is speaking to whom and in what setting, not by the word itself. The reader has to ask which circle of brotherhood is in view: the household, the family, the nation, or a bond of friendship.