Rothschild Trilogy Series by Carola Dunn

Lord Roworth, now working as an agent for Jacob Rothschild, finds himself in Brussels on the eve of Waterloo. He's sharing lodgings with a penniless soldier, Frank Ingram, and his pretty sister Fanny, not to mention their orphaned fosterling, Anita. When Frank is badly injured in battle, Roworth is torn between his duty to Rothschild and an urgent need to help Fanny get her brother back to safety in England.

Despite their different backgrounds, Lady Constantia Roworth loses her heart to Captain Frank Ingram, recuperating from his wounds at her father's estate, and is delighted when a surprise inheritance transforms the penniless soldier into an acceptable suitor. Original.