From CheshireRO's photostream on flickr; a fiber on her left arm was removed along with several spots in the background with Photoshop.
Lady Elizabeth's dress looks Napoléonic until the sheer outer sleeves, s style that was prominent in the 1820s. The skirt appears to have the early 20s A-line.
Keywords: 1816, Cousins, Lawrence - Thomas, Elizabeth Mary Leveson-Gower, Marchioness of Westminster, Countess Grosvenor, Leveson-Gower family, Grosvenor family, Marquise, British, curly coiffure, bun, partlet, sweetheart neckline, bodice, square neckline, ruff, dual upper puffed sleeves, arm bands, Empire waistline, waist band, close skirt, pin