A Lady of Persuasion by Tessa Dare

Only one thing could convince Sir Tobias Aldridge, an incorrigible libertine, to profess undying fidelity to a woman he’s just met. Revenge. What better way to get back at an enemy than by stealing the scoundrel’s sister? Not that Toby finds it a chore, seducing Isabel Grayson, a beguiling, sultry beauty freshly arrived from the West Indies. Isabel, for her part, is determined to reform the charming devil and get exactly what she craves: society’s respect. But it’s a dangerous gamble. For if Toby wins this battle of persuasion, Isabel could lose her reputation as well as her heart.