If the Duke Dares by Darcy Burke

Incorrigible flirt Acton Loxley, Duke of Wellesbourne is in need of a duchess. En route to greet a potential bride, he encounters a most intriguing and captivating widow who completely distracts him from his task. However, she disappears before he can deepen their acquaintance. Acton begrudgingly goes to meet his bride, unaware she and the widow are one and the same.
When Persephone Barclay's younger sister is compromised, Persephone must wed before the scandal spreads. Her parents scramble to present her to the Duke of Wellesbourne; except he's precisely the kind of rogue she has now vowed to avoid. Taking flight to avoid a match, Persephone runs straight into her would-be betrothed and pretends to be someone else. But dash it all, the duke proves to be irresistibly charming! If she remains in his company, she'll end up in the wicked scoundrel's clutches.
Even worse, her second attempt at escape lands her in increasingly poor circumstances. With nowhere to turn and her safety at risk, she may have to accept help from the one man who threatens her resolve-and her reputation.