Lady Be Bad by Megan Frampton

Once upon a time, a duke had five daughters who never made a stir. They practiced their French, their pianoforte, and their dancing…until one ran off with the dancing instructor, and the rest were left to face the scandal.

Lady Eleanor, the Duke of Marymount’s eldest daughter, knows the burden is hers: she must marry well to restore the family name. So a loveless match is made and her fate is set. But then Eleanor meets her intended’s rakish younger brother. With his tawny hair, green eyes, and scandalous behavior, Lord Alexander Raybourn makes her want to be very bad indeed.

With his very honorable sibling too busy saving the world to woo Eleanor, Alexander is tasked with finding out her likes and dislikes for his elder brother. But the more time he spends with the secretly naughty Eleanor, helping her tick off all the things on her good list for being bad, the more he knows what they want, and need, is each other.