Complex sentence: Before her marriage in 1826 she had lived in Florence, and had attended the receptions of the Countess of Albany, the widow of Charles Edward, the Young Pretender; and in London she had great influence in social and political circles.
Split into simpler sentences: Before her marriage, she had lived in Florence and had attended the receptions of the Countess of Albany, widow of Charles Edward, the Young Pretender. In London, she had great influence in social and political circles.