Incognito — Suzanne Allain

INCOGNITO is a historical romantic comedy set in Regency England that should appeal to fans of Jane Austen, Georgette Heyer, or Oscar Wilde. From the back cover:

Can a young lady allow her beloved sister to be sacrificed on the marriage altar? Emily Smithfield cannot. So when her mother announces that a marriage has been arranged between Emily's older sister Lydia and Lord Wesleigh, a man the sisters have never met, Emily offers to marry the gentleman in her sister's stead. Emily's act of selflessness leaves Lydia free to marry the man she loves and Emily the bride of a handsome young marquess. Not a bad bargain at all!

Lord Wesleigh, who knows nothing of Emily's matchmaking scheme, arrives in the small village where the Smithfields reside disguised as a curate, in an attempt to observe his betrothed anonymously. When Lord Wesleigh finds himself more attracted to Emily than her sister, the rules of the game change. Can he make Emily fall in love with him when she believes him to be a penniless curate?