More or Less a Countess by Anna Bradley

Finding a worthy husband should be simple for three beautiful debutantes in Regency London. But the Somerset sisters have a way of making it delightfully complicated…
Violet Somerset has always preferred a library to a ballroom, but to please her grandmother she agreed to one London season. With nothing to show for it but heartbreak, she’s reconciled to spinsterhood. Until a notorious rogue known as the Devil of Dare requests an introduction to her timid, gentle youngest sister, Hyacinth. Violet will do anything to stop the match—even if it means posing as Hyacinth herself.
Nicholas Balfour, the Earl of Dare, is enjoying the life of a rake far too much to take a wife.

But he must keep his promise to his grandmother. He’ll simply choose the meekest bride he can find, install her at his estate, then carry on as before. Hyacinth sounds perfect—until he discovers that her sweet demeanor hides the tongue of a viper and the mind of a bluestocking.
As Violet’s ruse threatens to unravel, however, she may find the tables turned. Soon she may have no choice but to tolerate her handsome fiancée—or fall in love with him.