The guardian - Michelle Zink

Without the Keys, something terrible will happen. Something that cannot be undone.
And with them, I might bring an end to the riddle of the Prophecy and my strange part in it.
If Alice and I are on conflicting sides of the Prophecy, the Keys would be dangerous in her hands.

Which means I have to find them.

And I have to do it before my sister.

This is the story of sixteen-year-old Lia Milthorpe's quest to discover her role and her twin sister's in a powerful prophecy that has affected twin sisters for generations. But nothing can prepare her for what she discovers - about herself, about her family, and about the danger that goes from haunting her dreams to becoming her reality.