Vanishing Girls By Lauren Oliver

Dara and Nick used to be inseparable, but that was before the accident that left Dara's beautiful face scarred and the two sisters totally estranged.

When Dara vanishes on her birthday, Nick thinks Dara is just playing around. But another girl, nine-year-old Madeline Snow, has vanished, too, and Nick becomes increasingly convinced that the two disappearances are linked. Now Nick has to find her sister, before it's too late.