The Duke and the Lady by Jessie Clever

Lady Louisa Darby blames herself for her mother’s death when the Darby sisters were still so young and has dedicated her life to ensuring her sisters’ happiness. So when she’s lured into a compromising situation that will endanger her youngest sister’s chance at a happy marriage, Louisa will do whatever is necessary to protect her family’s reputation and save her sister’s chance at the future she deserves.

Even if it means marrying the Beastly Duke.
Sebastian Fielding, the Duke of Waverly, has spent years cultivating his reputation as the Beastly Duke to ensure he’ll never make the mistake of falling in love and repeating his father’s fatal mistake. But when honor calls on him to marry Lady Louisa Darby, his best intentions may not be good enough. For not even a beast can withstand the charms of the belle of the ball.