If you thought being in the friendzone was painful, try imagining what it’s like in the relativezone. The OP had a major crush on someone, who had taken her out on multiple occasions to all sorts of events. One might think that they were dating, as she was hoping to with the guy one day.
Then, out of the clear blue sky, he drops the statement every hopeful girlfriend dreads hearing. He introduced her as “a girl I wish was my sister.” So, he pretty much closed the door on the two of them ever having a romantic relationship together. This isn’t Alabama, after all.