bro..talk to her in a gentle way..that what she feels is just a brotherly love vice versa...(does she has a big brother though?)...tell her that when she totally become an adult what she feels as of today will be vanished....feel it to her also that you care for her as a brother......
ex.
you: you know what sweety....ur cute and sweet and your like a little sister to me....what you feel towards me is a brotherly love the same way i feel about you....you know what..when i was at your age..i do have crushes to the girls older than my age..and surprisingly..that feeling was gone and its pretty normal....