What is Brother?

Legal Definition
Domest. relat. He who is born from the same father and mother with another, or from one of them only.

2. Brothers are of the whole blood, when they are born of the same father and mother, and of the half blood, when they are the issue of one of them only.

3. In the civil law, when they are the children of the same father and mother, they are called brothers germain; when they descend from the same father, but not the same mother, they are consanguine brothers; when they are the issue of the same mother, but not the same father, they are uterine brothers. A half brother, is one who is born of the same father or mother, but not of both. One born of the same parents before they were married, a left-sided brother; and a bastard born of the same father or mother, is called a natural brother. Vide Blood; Half-blood; Line; and Merl. Repert. mot Frere; Dict. de Jurisp. mot Frere; Code, 3, 28, 27 Nov. 84, praef; Dane's Ab. Index, h. t.
-- Bouviers Law Dictionary
Legal Definition
One person is a brother "of the whole blood" to another, the former being a male, when both are born from the same father and mother. He is a brother "of the half blood" to that other (or halfbrother) when the two are born to the same father by different mothers or by the same mother to different fathers. In the civil law, the following distinctions are observed: Two brothers who descend from the same father, but by different mothers, are called "consanguine" brothers. If they have the same mother, but are begotten by different fathers, they are called "uterine" brothers. If they have both the same father and mother, they are denominated brothers "germane."
-- Black's Law Dictionary
Legal Definition
A brother is a male sibling.
-- Wikipedia