One of my friends is flirting with me, but I view her like family. How can I explain this to her?

Bare in mind, she and I have known each other for about six years. During that time we've constantly acted like and refered to each other as brother and sister. Literally, we have friends who are shocked to realize we're not biologically related. Anyway, long story short she's recently started hitting on me. I know, I know...guys can misunderstand friendliness for flirting but I've known her for six years, I know which she's doing and when she's doing it. I want to explain to her that this just feels weird coming from her, but to do it in such a way that I can still retain the same type of friendship we've enjoyed having. Anyway have any suggestions?