Mary Radclyffe wears a décolleté dress with extensive lace trim in this work by Larkin. This is the image posted in Wikipedia detail-enhanced by gogm to bring out details of the rolled sleeves and skirt.
Her gown is black, suiting Puritan tastes, but tres décolleté.
Thepeerage has an article about her: "Mary Radclyffe is the daughter of Sir John Radclyffe. She married Sir John Stanhope, son of Sir John Stanhope and Catherine Trentham. Her married name became Stanhope."
The Wikipedia article for William Larkin is here.