>I have no interest in her whatsoever romantically

That's the most important bit. If you remain clear on that, then things can't go too wrong. There's nothing worse than someone sending mixed signals, it can drive the other person insane.

I think the best would be to have a clear conversation on the topic, explaining her that you consider her as a kind of sister, and that a romantic relationship just is not going to happen. You can reassure her that you'll always be there to support her, but that you can't tolerate her interference with your girlfriend. You understand that it's not easy for her, but that's just off limits. Maybe it could also soothe her to say that she's a special person to you, and that although love relationships come and go, your bonds can endure. At the same time, you must also realize and accept that this is probably a transitional phase, and when she comes out of it, she will probably be more distant from you. That's life, must happen, and is healthy, even if sometimes painful.