I'm looking for a story that I read back when I was a child. Around 2005, maybe? I recall the book seeming pretty old, but it was from a public library, so it could be simply deteriorated by usage.
The world is being afflicted with a mysterious sickness that has no cure and no one knows where did it come from (it is called gray fever, gray plague or something like that).
The story narrates two young princesses, the elder one would be brave and adventurous, while the younger one would be shy and more delicate. They both fantasized with a future where the younger one would fall sick and the elder one would depart trying to find a cure to save her.
But in reality, the elder one falls sick and the young one is forced to depart. I recall some sort of mage/warlock that helps her giving her a pair of boots that would let her walk big distances almost instantaneously. I also recall the younger one being trapped by a dragon, and in the end the elder one dies from the sickness, but is saved being transformed in a fairy. I also think the sickness is eradicated in the end.