How do I let this girl down who's into me that I don't feel the same way?

I used to work with this girl for like 5 years. We're friends outside of work and she used to date a mutual friend of ours, but they broke up a few months ago. A number of years ago I thought maybe she liked me, but lately not really. She emailed me the other day saying that she was into me. She said that it wasn't some stupid crush and that she's felt this way for years. She said that the past little while she said we were like brother and sister and stuff like that as self preservation.

I'm not really into her. What should I tell her to let her down easy? Or should I try nailing her first before telling her?