Henrietta Ponsonby, Countess of Bessborough
(16 June 1761 – 11 November 1821),
Henrietta Ponsonby, Countess of Bessborough (16 June 1761 – 11 November 1821), born Lady Henrietta Frances Spencer (generally called Harriet), was the wife of Frederick Ponsonby, 3rd Earl of Bessborough, and mother of the notorious Lady Caroline Lamb. Her father, John Spencer, 1st Earl Spencer, was a great-grandson of John Churchill, 1st Duke of Marlborough. Her sister was Georgiana Cavendish, Duchess of Devonshire.She was the great-great-great-great-aunt of Diana, Princess of Wales.
You can read more on her Wikipedia page