Who by Fire by Diana Spechler

Bits and Ash were children when the kidnapping of their younger sister, Alena—an incident for which Ash blames himself—caused an irreparable family rift. Thirteen years later, Ash is living as an Orthodox Jew in Israel, cutting himself off from his mother, Ellie, and his wild-child sister, Bits. But soon he may have to face them again; Alena's remains have finally been uncovered. Now Bits is traveling across the world in a bold and desperate attempt to bring her brother home and salvage what's left of their family.

Sharp and captivating, Who by Fire deftly explores what happens when people try to rescue one another.