Comment by wildgift

It's not that simple. I think it originated in surfer culture, as a kind of appropriation of Black American culture to call each other brother and sister. In the 90s, on the west coast, it was common among white hippies, deadheads and surfers to call each other "brah."

That said, I don't know where African Americans got it from, but labor unions used the terms brother and sister going back to their beginnings in the 1800s. So it may have been copied from that.

Anyway, I think it migrated from surfer culture to fratboy culture. There, it acquired a tinge of elitism and sexism, because... well, frats are elitist and sexist.