The Convenient Marriage by Georgette Heyer
Horatia Winwood is simply helping her family. When the Earl of Rule proposes marriage to her sister Lizzie, Horatia offers herself instead. Her sister is already in love with someone else, and Horatia is willing to sacrifice herself for her family's happiness. Everyone knows she's no beauty, but she'll do her best to keep out of the Earl's way and make him a good wife. And then the Earl's archenemy, Sir Robert, sets out to ruin her reputation... The Earl of Rule has found just the wife he wants Unbeknownst to Horatia, the Earl is enchanted by her. There's simply no way he's going to let her get into trouble. Overcoming some misguided help from Horatia's harebrained brother and a hired highwayman, the Earl routs his old enemy, and wins over his young wife, gifting her with a love that she never thought she could expect.