The Thief Lord (Chicken House Novels)

Two orphaned brothers, Prosper and Bo, have run away to Vencie, where crumbling canals and misty alleyways shelter a secret community of street urchins. Leader of this motley crew of lost children is a clever, charming boy with a dark history of his own. He calls himself the Thief Lord.
Prosper and Bo relish their new "family" and life of petty crime. But their curel aunt and a bumbling detective are on their trail. And posing an even greater threat to the boys' freedom is something from a forgotten past: a beautiful magical treasure with the power to spin time itself.