Her article in thepeerage is here. She married Admiral Lord Hugh Seymour and had six children. She is the Waldegrave sister to the right in this image.
From the Christie's Web site.
This portrait was probably painted about when the youthful-looking Lady Horatia, b. 1762, came of age. She wears a bouffant 1780s coiffure under a cap wide enough to enclose the 'do.
Keywords: Bone, Anne Horatia Waldegrave, Lady Anne Horatia Seymour, Waldegrave family, Seymour family, bouffant coiffure, cap, neck band, cowl, scoop neckline, tight sleeves