Hmmm. I've been in this situation. Twice as the person being fancied, and quite a few times as the "friend-zoned" guy.
The one thing I've noticed over and over and over again, without fail. When the penny finally drops and the person realises that there really, really is NO chance of a relationship happening,... they get bitter. Few things sting more than realising you've being fooling yourself. And once you get over that you start noticing how much energy you were putting into the relationship, and how little they did. But of course that's not THEIR fault, they always said they weren't interested. But you remember. All the times you did what they wanted and not what you wanted to do. All the times you made the effort to call them, email them, go to their house, buy them presents. And how that compares to all the times they didn't do these things for you.
Resentment. As I said I've been on both sides so I know how unfair it is to be resented when you've done nothing wrong, and also how hard it is NOT to resent the person you've been pursuing.
Right now, even though you're talking about this other girl, she thinks she's got a chance to be with you. That you'll one day realise that she's the one for you. If you're 85%-90% sure she fancies you, then 100% sure she does. Right now she'll put up with anything from you. Including talking about being in love with other women. The only things that hurt her is anything you say to her which ruins her dream of being with you. Like "You're like my big sister". That's always great to hear from someone you're having fantasies about.
A good lithmus test for yourself is figure out how often you call/text/email her, and how often she does for you. Who initiates conversations. Do you find yourself talking a lot about yourself to her. Who's doing the legwork in your friendship.
If you're not interested in having that kind of relationship with her the best and kindest thing to do is to either start distancing yourself (which will probably upset her unless you have a good explanation), or have "the conversation" and then take a break from each other. And that conversation is awful but goes "I get the feeling you'd like us to be more than friends. If that's true then it's cruel for us to hang out when I don't feel the same way....blah blah blah....we'll always be friends". Which is bollocks but everybody always says it.
Right now if you start dating this other girl, and she has her realisation that you're not interested in her, and that you possibly "used" her, just remember "hell hath no fury like a woman scorned". There's a reason why it's a saying.