While this appeared on eBay; the UK National Portrait Gallery has this image listed as, "Sibell Mary Grosvenor (nee Lumley), Countess Grosvenor (later Lady Wyndham).”
Lady Sibell’s dress appears to pre-date the 1894 publication date based on the lack of leg-o-mutton sleeves. The square neckline, highlighted by a ruff, is surrounded by a delightful set of plain and lace flounces.
Keywords: W. & D. Downey, Sibell Mary Grosvenor (nee Lumley), Sibell Mary Grosvenor (nee Lumley), Countess Grosvenor, Lady Wyndham, Countess, British, curly coiffure, square neckline, neckline ruff, modesty piece, bertha, lace, tight sleeves, cuffs, necklace, earrings, brooch
Her article in the peerage is here.