Devil's Cub by Georgette Heyer
You elected to come to me and by God you`ll obey me, if I have to lay my whip about your sided! 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 gamesmaster called the Devil`s Cub. But when he is forced to leave the country, Mary Challoner discovers his friendish plan to abduct her sister. And only by daring to impersonate her can Mary save her from certain ruin. . .