What's the Name of That Book??? discussion

What I remember from the cover is a brown border with an illustration of two girls, one blonde & in a white dress, the other a brunette and in a brown dress. Title in gold or something fancy. I got this at a Scholastic book fair somewhere between 2007-2010, not sure when it wasn’t published.

There’s two sisters, one wants to go out and see the world but the other (main character) wants to stay at the castle. When the first one gets sick with something called the White Death, main character knows she has to go out and find the cure for her. Someone gives her a package with some tools in it, including a spyglass/binoculars-type thing but a lot more powerful, and some boots that can take her several leagues with one step, they’re adjustable, and I think a bag of food that never runs out?

There’s someone named Ryan, I think, and he can fly/levitate. There a scene in a forest where the heroine is concerned that he’s a specter because they can shape-shift and don’t leave footprints, since he’s floating he doesn’t leave a footprint.

She has an encounter with a flock of griffins, I’m pretty sure she defeats them by feeding them with her magic food bag.

She has to find a dragon to tell her the cure, I think the dragon she ends up meeting has a name starting with an S. Classic dragon, hoards cool stuff. Keeps the heroine in her cave, the heroine does some exploring obvi and find some cool clothes, I think. I remember her asking the dragon about the cure but the dragon wants to know who the cure is for and the heroine won’t tell her (?) so the dragon laughs and says “the cure for the white death is a swallow of milk”

I don’t remember much else, there might have been a waterfall at the end. Thank you so much in advance for any help!

The Two Princesses of Bamarre by Gail Carson Levine?

Bless your soul! Thank you so much!!!