Males who share a common parent(s), either natural or adopted.
Generally refers to males that share a common parent, but can also refer to a group associated, such as in a Fraternity, where members are referenced as 'fraternity brothers'.
In a connotation where there is no 'parental' or 'genetic' connection, the meaning refers to a close bonding, relationship than a simple association or friendship might imply. A greater sense of responsibility to the group, more like being part of an extended family setting.
Some ancient works depict the tumultuous relationships between siblings, however most are male - female associations. Most famous is Wagner's 'The Ring of the Nibelung'
Then there is also the Czech brothers who do Porn, Jirka and Karel Bartok, among others.
There is no 'hard' evidence to indicate that 'brothers' are more tempted to engage in sibling sex than brother and sister are. In fact, ancient rituals in some cultures insisted on the union between different sibling genders to propagate the family line, to insure its purity.
[Sexual] While many studies disprove the theory that a sexual attraction exists among brothers or siblings, most claim it is more a result of social and cultural influences rather than some genetic disposition.
The fantasy of brothers having sex exists, as some claim, is due to the need to have a deep bonding, that is associated with 'brotherly love or sibling sex' Sex being considered the ultimate form of showing one's affections, in a physical sense. Hence the growing trend towards sexual sites dealing with 'fraternity members' or other closely knit groups of males. It is an attempt to provide that 'close' association, 'bond' that some gays desire, but won't admit to.
The sexual attraction of 'brothers' is also rooted in the desire to test boundaries, or to rebel against the taboos of society. There is also the 'predatory' aspect of 'sibling sex' that can occur.
'Brother' sex is also about youthful exploits, where the sex is merely physical in nature (see 'Boys') which implies a sort of curiosity being explored, tested, and who better to do such experimentation with, than someone close, who you can trust. Whether a 'brother' in the sense of being a blood relation or of a group relationship, such as a Fraternity.
Generally used to denote a closeness of a group, in naming conventions. Example being 'brother records' or 'blues brother's' where the term 'brothers' is used to show a closeness and mutual deep affection for the subject, such as 'blues' or other areas.