Her article in thepeerage is here. She was Dorothy, Countess of Sunderland’s mother.
From artuk.org; enlarged one quarter. Also posted in the Collars to Casual - 1630 to 1640 Album here.
Here is another van Dyck portrait of a Lady in a white satin dress with a high vee waistline wearing a curly coiffure.
Keywords: van Dyck, Dorothy Percy, Dorothy Sydney, Countess of Leicester, Countess, British, curly coiffure, hair jewelry, earrings, necklace, fitted bodice, trapezoidal neckline, brooch, clasps, elbow length notched puffed over-sleeves, lining, elbow length under-sleeves, lace-edged cuffs, girdle, shallow vee waistline, wrap