I looked for the location and the reliable site The Athenaeum states the location is not known.
There is an article about Sir Christopher Sykes and his children in thepeerage, but no mention of her.
From 18thcenturyblog.com/2012/03/george-romney?.
Lady Sykes wears a full-skirted dress with vee waistline. Her skirt is not wide enough to need substantial mechanical supports, but there may be some structure below to support the weight of her skirt and her over-skirt.
Keywords: 1786, Romney, British, straight bouffant coiffure, jeweled headdress, cap, chemise, scoop neckline, long tight sleeves, vee waistline, full skirt, over-skirt, shoes