The fairest of them all / Cathy Maxwell.

Enhanced descriptions from Syndetics:

The penniless orphan of a disreputable earl, Lady Charlene Blanchard thrives on the adventure of picking the pockets of unsavory gentleman to survive. But due to her extraordinary beauty and prized bloodlines, she is hand-chosen as a potential bride for the Duke of Baynton who is on the hunt for a suitable wife to provide heirs. All Char has to do is act the part she was born to play and charm a duke she's never laid eyes on into proposing. Except the duke turns out to be the tall, dark and sexy stranger who just caught her red-handed as a thief!



Or is he



Jack Whitridge is the duke's twin who had "gone missing" over 10 years ago. Now back in England, he knows that the supposed Lady who has his brother's heart is hardly duchess material--except he needs her to save his adopted country from war. He is willing to bargain with her heart, until he finds himself falling for Char himself. . . .