To turn this situation into a good ending,here's what I think you should do..
Sit her down(preferably in a quiet place) and tell her exactly how you feel about her,and that you see her more like a sister than a would be girlfriend.
Truth hurts but she should accept it,but if not and she starts crying,lend her a shoulder to lean on.
Once she calms down,tell her,even though you can't be her boyfriend(if that's what she wants),you can always be her best "male" friend.