hahah to be frank when i tell a guy that he's like a 'brother' to me, i don't really mean it in that sense. it's a subtle way of rejection, for me anyway. but we could always remain friends if he doesn't mind. i still hang out with guys who used to express interest in me. but i don't think i'll ever see them beyond a platonic relationship.
i know someone who's like you. he has many female friends but none of them are interested in him romantically. he also always gets the 'you're like a brother to me' excuse. i'd call it an excuse anyway. he knows it too. and he's frustrated things never progress the way he wants it to.
my advice to you is not to give up (if you really think she's the one for you), especially since u guys are already such buddies. just be your usual nice self, be a good friend and enjoy her company without getting your hopes too high up. sometimes persistence do pay. it all depends on the girl and this thing called fate