While the Bettesworth and Sargent families are acknowledged in thepeerage, neither Charlotte nor John are listed.
From bbc.co.
Charlotte Bettesworth wears a white dress, a cap or turban with a gold braided extension, small lace cuffs, and a small brooch to convey status. Her hair is un-powdered. Her dress could be a sheath.
Keywords: 1776, Romney, Charlotte Bettesworth, Mrs. John Sargent, high straight coiffure, cap, modesty piece, square neckline, long tight sleeves, cuffs, lace, waist band, full skirt, brooch