The Duel by Barbara Metzger

If only he could go back in time. Then Ian, Karl ol Marden, would not have dueled, and he would not have hit an innocent bystander -- a sickly lad, no less. And he certainly would never have brought the boy's sister, Athena Renslow, to stay at his house without a chaperone.

On top of it all, the nineteen-year-old beauty is under the mistaken impression that Ian is the gentle hero who rescued her brother instead of the rogue who practically killed him. So, in order to keep the Renslows' lives from getting any worse, Ian resolves to marry the girl. But, Athena won't let the wealthy, handsome bachelor sacrifice everything for a country nobody, despite the urgings of both their families--and their hearts.