Catherine, Lady Stepney by John Hayter (National Portrait Gallery - London UK)
The only biographical information I can find for her is here. According to this article, "...was first married to Russell Manners, and under that name published two novels, 'Castle Nuovier, or Henry and Adelina,' 1806, and 'The Lords of Erith,' 1809. On 8 June 1813 she married, at Edinburgh, Sir Thomas Stepney...Sir Thomas died on 12 Sept. 1825 (Gent. Mag. 1825, ii. 277). His widow, who was pretty, accomplished, and fashionable, and made her house the rendezvous of authors and artists, died at Henrietta Street, Cavendish Square, London, on 14 April 1845. She wrote in later life four novels: 'The New Road to Ruin,' 1833; 'The Heir Presumptive,' 1835; 'The Courtier's Daughter,' 1838, 1841; and 'The Three Peers,' 1841..."
She divorced her first husband. Her second husband's family is discussed here. It seems she was more famous than he.
John Hayter's Wikipedia article is here - he visited and painted portraits in Hawaii.