She skipped on her husband, a member of the Villiers family, and ran off with Sir Robert Howard. She was imprisoned for adultery and escaped. Her article in thepeerage.com, including this image, is here.
Frances' dress is a complex group of vee lines beginning with a plunging modesty piece, then a fichu-like covering, then a curved-vee set of lapels, vee markings on her bodice, and finally a vee waistline. She wears typical tabbed and slashed sleeves.