The Virgin and the Viscount by Robyn DeHart

As a Lady of Virtue, Matilda Brooks swears to reform the most despicable man of her acquaintance, her brother-in-law, Sullivan Chase, Viscount Glenbrook. Well he may not be the most despicable, but he is certainly arrogant, flirtatious, and entirely too charming. To make matters worse, he has the irritating tendency to poke fun of her and rile her emotions as no other man does. However, when she confronts him, he laughs off her concern about his slothful ways.
But when a carriage accident forces Sullivan to play knight to Tilly’s damsel, his unexpected act of chivalry ends up costing them both their freedom. Her compromised reputation and his honorable declaration forces them into a marriage neither of them wants. Which is most inconvenient, given that she has sworn to despise him forever.