The Vanishing Moment
This book reminded me strongly of Margaret Mahy's magical novels, The Changeover and The Tricksters -- the endangered little brother, the coastal setting, questions of fate and free will, and young women at the centre. But instead of concentrating on the magical element, The Vanishing Moment takes its time setting up the initial scenario -- Arrow's emotional paralysis and her encounter with muggers, Marika's horrifying loss. The question of the Interchange doesn't even arise until the final quarter of the novel. After this, events swirl rapidly to a punchy conclusion.
I'm not enjoying much YA at the moment, but I did sprint through this and the last quarter of the book was a great reward for the slow start.