Brüderchen und Schwesterchen (2008)

Little Brother and Sister's new mother is not at all as humble and kind as she pretends to be, they are mistreated by their wicked stepmother and their new stepsister, so much that they decide to run away from home, after she poisoned their father to inherit his money. But the stepmother, who is also a witch, enchants the streams of the forest where the children are wandering, so that drinking from them would turn them into animals.