Her terse article in thepeerage is here, just beneath her husband's terse article.
From Wikimedia.
Lawrence appears to have merged her fichu and bodice into a mass of white. Lady Simeon appears to be wearing a waist band decorated with diagonal gold stripes (that have become zones for cracks to develop).
Keywords: Lawrence, Rebecca Cornwall, Rebecca Lady Simeon, Cornwall family, Simeon family, British, bouffant coiffure, cap, fichu, vee neckline, bodice, square neckline, waist band, full skirt, wrap, fur