This World So Fierce by Mary Bourque Marcotte

Trish and Nikki are orphans who live on the poor side of town. But they know what it means to have a family that cares, even if that family is not blood. Their foster parents can't have children of their own, so they raise children who come to them from a world of pain and loss. When the family is asked to take in one more, they can't refuse. But it might be one more than they can handle.