Mike, any time a girl tells you that she thinks of you as a brother, it usually means she wouldn't consider anything else other then friendship. There's really not much you can do but be her friend, if you can still do that. In the long run, things might happen if she decides to break up with her b/f however you have to decide whether you can stand being around especially with the way you feel about her. If you cannot, it's better that you move on simply because it's gonna hurt a lot less now. You don't want to force her to do anything and she's obviously with him so you have to respect that no matter how much you think is wrong.
-to be nobody but yourself in a world which is doing its best, night and day, to make you everybody else means to fight the hardest battle which any human being can fight; and never stop fighting.- e.e.cummings