Sir Thomas Moore’s article in thepeerage does not mention her, but her Wikipedia article is here. According to the Wikipedia article, she was a foster, not adopted, child. She is at the far left in the More family portrait.
From Peter's photostream on flickr. Also posted to the Early 1500s - 1470 to 1550 Album here.
She is wearing a vest-like upper bodice, possible her partlet above a low-cut bodice with probable long close sleeves. Her hair is probably covered by the inner layer of an English hood.
Keywords: 1527, Holbein - Hans the Younger, Margaret Giggs, Margaret Clement, Giggs family, More family, Clement family, English, straight coiffure, English hood, lining, choker necklace, necklace, vest partlet, high vee neckline, lapels, fitted bodice, close sleeves