Well speaking from personal experience, if you ignore her and act like she doesn't exist, that should work. Sure, it may make her hate herself and you in the long run but I don't think I'm incorrect to state that rejection happens to men and women of all ages. Call me harsh but I feel it's just the facts of life. If you tell her you like her as a sister or what not, and she's indeed
that infatuated with you, there's a good possibility she'll think she still has a chance.
What do I know.