Jakob II. (VII.) Stuart hatte mit Catherine Sedley drei Kinder: seine Tochter Catherine Darnley (um 1681-1743), seinen Sohn James Darnley (1684-1685) und seinen Sohn Charles Darnley, von dem keine Daten vorliegen und der ebenfalls als kleines Kind starb. Ihre Liebesbeziehung begann um 1678.
Catherine Sedley hatte ihren Geliebten mit einer weiteren Hauptmätresse von ihm zu teilen: Arabella Churchill. "They were both plain women. Charles [Karl II. Stuart] is said to have once remarked that he believed 'his brother [Jakob II. (VII.)] had his mistresses given him by his priests for penance' - a witticism especially directed against Catherine Sedley. This ugly but clever woman bore James [= Jakob II.] a daughter, and dominated him with bold vulgarity for many years. Catherine had more wit and discretion. Speaking of James's mistresses she is said to have exclaimed, 'Why does he choose us? We are none of us handsome, and if we have wit he has not enough to find it out.' James made her Countess of Dorchester, but she was so hated by the Duchess of York [Maria von Modena, Jakobs II.(VII.) zweiter Gattin] and the priests that she was obliged to leave England. Later she returned as the wife of Sir David Collyer [oder Colyear (1656-1730), erster Graf von Portmore], by whom she had two sons [David (1698-1728/9) and Charles (1700-1785)]. According to the gossip of the day, she once remarked to the latter, 'If anybody should call you sons of a whore, you must bear it for you are so, but if they call you bastards fight till you die for you are an honest man's sons.'" (in: Marie Ruan Hopkinson: Anne of England – The Biography of a great Queen, id., pp. 73-74).
"Where other royal wives had endured their husband's infidelity, Mary Beatrice [Maria von Modena] was not prepared to tolerate it. Barillon was following the unfolding scandal [im Jahr 1686] avidly and reported to Louis XIV: 'She loves her husband in all sincerity; she is Italian and very proud. Her grief manifested itself very plainly. ... She has openly declared she will not suffer the public scandal it is intended to establish, that she will not see the new Countess [of Dorchester], and that if the King does not separate from her, she will retire to a convent, in any country that may be.' The force of Mary Beatrice's resistance surprised James, and she left him in no doubt of his choice. Either he gave up Catherine Sedley, or she would carry out her threat to enter a convent. She summoned James to her bedchamber, where Father Petre and other priests confronted him. They pointed out to James the harm it would do the reputation of a Catholic king to be seen disporting himself with a heretic mistress. He conceded and Catherine was ordered to leave the country. She prevaricated and resisted, retorting that she was a freeborn Englishwoman. Eventually, she departed for Ireland - temporarily, for it was soon reported that a house was being prepared for her in St. James' Square ... (im Jahr 1696 heiratete sie David Collyer, den ersten Grafen von Portmore]." (in: Maureen Waller: Ungrateful Daughters – The Stuart Princesses who stole their father’s crown, id., pp. 41-42).