Devil's Cub by Georgette Heyer
The excesses of the young Marquis of Vidal are even wilder than his father's before him. Not for nothing is the reckless duellist and gamester called 'the Devil's Cub'. But when he is forced to leave the country, Mary Challoner discovers his fiendish plan to abduct her sister. And only by daring to impersonate her can Mary save her sibling from certain ruin. Georgette Heyer's wonderful characters and stories made her the undisputed queen of the historical romance, and an international bestseller, still loved today by a huge readership.