The Ghosts of London — Amy Cross

Two sisters, lost in one of the world's busiest and most dangerous cities. After running away from home, Katie arrives in London and immediately tries to find her sister Rachel. Unfortunately, she soon finds herself in trouble, and a stranger's helping hand might just make things worse. Meanwhile, her sister Rachel keeps dying over and over again, while a mysterious man named Robinson tries to work out why the River Thames has suddenly been drained of all its water.