Who said you have to wait until she's 13? Or is it a code you go by for your own preferences or views? (Like ''teens'' only) If not, a one year difference shouldn't be a problem at all for you. Now as for her brother, do you know him personally? I'm not sure if he's the typical over-protective brother, or has anything against you for liking her or wanting to have a personal relationship. Knowing him personality wise would determine if he'll flip out about it. Either way, there could be a solution, just answer my q's. =p
First,I have to wait because her parents don't like her being fooled around by boys.Second answer, I do know my cusin's boyfriend personally because we play basketball together and tell each other secrets, I like to think of him as a 1 year older twin because of our similarities.
You'd be in good luck if the information you got about her sharing the same feelings is true. Most the times at that age it's hard to admit with bravery, so your best bet would be to continue what you're doing. Especially if it's making her happy and enjoy your company. Also, depending on how long you've known each other could be a factor. There should be no major rush to get with her, take your time to better understand what she looks for and remember all the important big or small things that impresses her. (With a nice balance of being your self, of course.)
~Blade~