The Masqueraders by Georgette Heyer
This is the adventure story of the Prudence and her brother Robin. Robin and the Prudence Merriot were dissembled when they were children. To prevent from being captured, both brother and the sister have created the new roles for themselves. Robin created herself as Miss Merriot and prudence become the new and handsome young Peter.
In order to make the story more interesting, peter merroit marry to the old man named Anthony Fanshawe.
He was the most conceited, arrogant and the scheming rogue. But there is no one who rescues the rich Merriot from the abductor.