If I read Ask.com correctly, Catherine is to the left and she was visiting her home after marrying George Herbert, the 11th Earl of Pembroke.
Brullov documents mid 20s fashion showing two Ladies wearing dresses with pleated bodices, wide belts, full flounced skirts, and wide hats.
Keywords: 1824, Brullov, Vorontsov family, Galitzine family, Countess, Princess, Lady-in-Waiting, hurluberlu coiffure, ribboned hat, sweetheart neckline, modesty piece, partlet, neckline ruff, pleated bodice, dual high puffed sleeves, gigot sleeves, cuffs, belt, full skirt, flounces, maxi length skirt, gloves, shoes, handkerchief, parasol