Masqueraders by Georgette Heyer
Robin and Prudence Merriot have been dissemblers since they were children. And in this age of slippery politics, they need to be. So it is nothing for them to rescue a rich heiress from her abductor. But once committed to their masquerade, they must see it through... For more than fifty years Georgette Heyer was the undisputed queen of historical romance. And as these hugely succesful reprints have shown, she still has a large devoted readership.