From Wikimedia.
Lady Louisa appears to be wearing a black yoke bodice and a striped under-bodice and, surely, a chemise for modesty. There is a mystery here - she was born in 1745 and this portrait is dated to 1821, making her around 76. This ia a portrait of somebody who could be 16 or 26.
Keywords: 1821, Hoppner, Lady Louisa Manners, Tollemache family, Manners family, Countess, British, curly coiffure, scarf, hat, bows, trapezoidal neckline, modesty piece, under-bodice, vee décolletage, quarter length close sleeves, yoke over-bodice, criss-cross bodice, bows, cape, close skirt