Christie's lot notes for this follow, "Anna Horatia (d. 1801) and Charlotte Maria (1761-1801), daughters of James, 2nd Earl Waldegrave (1714/15-1763) and Maria, illegitimate daughter of Hon. Sir Edward Walpole. Anne Horatia married Lord Hugh Seymour in 1786 and Charlotte Maria married George Henry, 4th Duke of Grafton (1760-1844) in 1784."
Charlotte Maria's article in thepeerage is here.
The bouffant coiffures are as much 1780s as Air Supply songs are 1980s.
Christie's overview notes follow, "The Waldegrave Sisters: Lady Anna Horatia and Lady Charlotte Maria, wearing white dresses with frilled collar and bows at corsage, Horatia with white plumes in her long powdered curling hair and Charlotte with white bonnet tied with white ribbon under her chin; sky and cloud background signed and dated on the backing paper 'R.... Cosway R. A. Primarius Pictor Serenissimi Walliæ Principis Pinxit 1789' oval, 3 in. (76 mm.) wide, gold mount."
Keywords: 1789, Cosway, Anne Horatia Waldegrave, Lady Anne Horatia Seymour, Charlotte Maria Waldegrave, Duchess of Grafton, Waldegrave family, Seymour family, FitzRoy family, Countess, Duchess, bouffant coiffure, feathered headdress, ribboned hat, strap, bows, neck band, vee neckline, ruffles