Complex sentence: The new Duchess of Westminster was far wealthier than her three predecessors, the first of which, Constance Cornwallis-West, was the mother of the Duke's children, all of whom were older than the Duchess herself.
Split into simpler sentences: The new Duchess of Westminster was far wealthier than her three predecessors. The first of them, Constance Cornwallis-West, was the mother of the Duke's children, all of whom were older than the Duchess herself.