On My Lady's Honor — Kate Silver

ONE MAN'S PROMISE BECAME...
All her life, Sophie Delamanse cursed the twist of fate that made her a woman and denied her the same glory and adventure as her twin brother, a Musketeer in the King's Guard. Envisioning only a dull future marriage to Count Lamotte, one of her brother's Musketeer friends and a man she'd never met, Sophie's life suddenly took a tragic turn when plague swept through her village, consuming her family in its deadly wake. Her brother's last words promised that Lamotte would come for her, but when he never arrived, Sophie vowed to go to him--disguised as her brother--and make him pay for his dishonor.

...ONE WOMAN'S DESTINY
Glad to see that his comrade-in-arms had returned from the plague-stricken countryside unharmed, Lamotte was confused by his quarrelsome behavior. But that was before he discovered that underneath the Musketeer uniform was Delamanse's spirited sister Sophie. Explaining why he never arrived to rescue her was difficult enough; convincing her to give up her charade in the King's Guard was impossible. If she didn't want marriage now, the least he could do was keep her safe--until he realized that he wanted far more than that...he wanted her heart.