Probable Sons by Amy Le Feuvre

Being a confirmed bachelor, Sir Edward Wentworth is horrified when his young orphaned niece arrives. Yet gradually over the coming months he becomes very fond of Milly, as he listens to her bright chatter about her life at his house and the people she meets, particularly the Maxwells who live in the forest. Through Milly's complete trust in God, Sir Edward is made aware of how far he has grown away from Christianity, and he reflects on this further when recovering from an accident. Milly's favourite story is that of the Prodigal Son, which is given special meaning by the return of the Maxwells' missing son. When Sir Edward nearly loses Milly forever, he realises that he has the courage to turn away from his previous life, and is himself a Prodigal Son.