Hmmm... It's a bit of an excuse, but not entirely. Sorry :/ Basically, what happens is that they think you are a perfectly nice guy, but they just don't feel 'that way' about ya. So in that respect, the 'brother' or 'just a friend' thing is true.
However, if I really liked a guy, and I was friends with him first, that would only stop *me* from doing anything - if he'd tell me he liked me, I'd go for it, and not say that 'I wouldn't want to risk the friendship'.
Edit: I suppose I'd just keep hanging out with her. Will she change her mind? Probably not. I'd also recommend going out with other could-be-more-than friends. Perhaps she'll realise what she could miss out on
Don't get your hopes up though, try and focus on other girls a bit more. It could even help your case with this girl, and if not, you may find another possible future girlfriend