To Tempt a Saint (Sons of Sin #1)
Kate Moore

Despite being knighted for saving the Prince Regent's life, Sir Alexander Jones will never truly be accepted by the nobility. But if he's to find his long lost brother, Xander must win the first willing heiress he can find— and obtain the funds he needs to search London's darkest corners.
Slandered and denied her rightful inheritance by the machinations of her unscrupulous uncle, Cleo Spencer has been living on a farm with only a meager stipend to support her and her brother. Only by marrying can Cleo access her funds and provide her younger brother the life he deserves.