Complex sentence: One memoir was an account of her mother's life; another memoir was about the fortunes of her mother's family and the last memoir recorded the misfortunes (casus suorum) of the family of Agrippina and Germanicus.
Split into simpler sentences: One memoir was an account of her mother's life. Another memoir was about the fortunes of her mother's family and the last memoir recorded the misfortunes (casus suorum) of the family of Agrippina and Germanicus.