Similar to the friend zone, but much, much worse. Simply put, it is when the girl you have feelings for cares about you to the extent that she thinks of you as not just a friend but as her "brother", making any potential chance for a relationship disappear.
Guy 1: God, I'm in the friend zone...
Guy 2: Shut the hell up, at least the friend zone has a way out. I'm in the brother zone. I'm totally fucked.
Guy 1: Damn, and I thought I had it bad.
Guy 2: You ain't kidding.