Looking for a fantasy novel about a wizard whose nephew is kidnapped by a family. The son in the family is cursed and they need the wizard's nephew because something about him can break the curse. The wizard goes to save his nephew, but ends up taking the cursed son with them when they escape. The wizard then starts to fall in love with the cursed son (who is a grown man) and then has to make a difficult decision about whether or not to return the cursed son to his family. One of the main characters might have been named Cassius or something beginning with a "C". The only other thing I remember is that the writer was an Australian woman.