From thepeerage.
It looks like Anthony Russell is following Beaubrun brothers style and Lady Mary is following French style for both hair and dress in this portrait.
Keywords: Russell - Anthony, Mary Villiers, Duchess, British, Villiers family, Stuart family, hurluberlu coiffure, hair jewelry, off shoulder scoop neckline, chemise, modesty piece, puffed over-sleeves, slashed sleeves, necklace, jeweled bodice ornament, clasps