This portrait was misidentified as the subject's daughter, Lady Sarah Sophia Fane, later Viscountess Villiers, and then Countess of Jersey. It is probably a portrait of her mother, the Countess of Westmorland, who was also named Sarah. The error is apparently written on the back of the item itself, as cited in the linked source, which is an art sale catalog that identifies the sitter as "Sarah Villiers, Viscountess of Jersey" (no such person ever existed). The date of the portrait is given as 1786, which is probably correct based on clothing style. Sarah Fane, the daughter, was born in 1785, and the portrait looks similar to other portraits of Lady Westmorland, who died in 1794.