Liesl and Po by Lauren Oliver

This book has a number of elements (evil stepmother, orphans, two boxes that get confused for each other) many readers will find familiar, especially for a young adult story, but they come together in a creative and satisfying way. Despite all the elements of adventure and mystery (although the solution to the mystery's clear from the moment you realize that something mysterious is going on...), the really strong parts of the book are the ones about family and dealing with moving on after someone has died. This book does a beautiful job dealing with death and a sad world with just the right touch of gravity.