Sharpe's Escape by Bernard Cornwell

Sharpe's Escape: Richard Sharpe and the Bussaco Campaign, 1810 (Richard Sharpe's Adventure Series #10) - The British army is moving south to Lisbon and destroying everything in its path to starve the French Army.  Sharpe's commanding officer gives command of the light company to his brother-in-law. Along the way Sharpe encounters two brothers, Ferragus and Major Ferreira, who are scheming with the French to save their wealth.  Trapped in the brother's warehouse with Sergeant Harper, Portuguese officer Jorge Vicente, and English governess, Sarah Fry.