Sophia is the daughter of siblings Lady Isabella Fitzwilliam, Heiress of Blayne and Lord Harcourt Fitzwilliam, Marquess of Blayne. Isabella was raped by her brother, who then left for France. 
Unbeknownst to him, Isabella became pregnant and gave birth to Sophia while he was away. 
In order to protect her daughter, Isabella had Sophia sent away to a boarding school where she was well taken care of, but completely ignorant of her parentage. 

At 15, returned to live with her mother as her ‘ward’.