Complex sentence: She was the only daughter of the 3rd Earl of Bessborough and Henrietta, Countess of Bessborough, and related to other leading society ladies, being the niece of Georgiana Cavendish, Duchess of Devonshire, and cousin (by marriage) of Annabella, Lady Byron.
Split into simpler sentences: She was the only daughter of the 3rd Earl of Bessborough and Henrietta, Countess of Bessborough. Her social credentials also included being niece of Georgiana Cavendish, Duchess of Devonshire, and cousin (by marriage) of Annabella, Lady Byron.