Flirt with her. Ask her out. Specifically use the word "date" if you are asking her out. Tell her that you are attracted to her.
The age group you are talking about is difficult because you get so many emotions bombarding you that you don't really know how to react. She may be saying she thinks of you like a brother because she doesn't really know what else to call her feelings for you.
I will say though, that normally, if a girl starts saying a guy is like a brother to her, chances are that is her nice way of saying that she is not into you romantically.
Good luck.
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