Her article in thepeerage is here while her article in Wikipedia is here. She bore a son by her husband, Francis Seymour-Ingram, 2nd Marquess of Hertford and "She and George IV Hanover, King of the United Kingdom were associated between 1807 and 1819."
From hoocher.com/Joshua_Reynolds. This was the best image I could find, but I enlarged it one quarter.
Mrs. Seymour-Ingram wears everyday dress under a large patterned wrap. She also wears a sheer brown-tan wrap. This high feathered coiffure suggests that this ensemble is probably meant for indoor wear.
Keywords: 1777, Reynolds, Isabella Ingram-Shepheard, Isabella Seymour-Conway, Isabella Anne Seymour-Conway, Marchioness of Hertford, Ingram-Shepheard family, Seymour-Conway family, Marquise, British, mistress, King George IV, high straight coiffure, jeweled feathered headdress, wrap, long tight sleeves, close skirt, fan