Complex sentence: At the age of sixteen she married Cornelis de Graeff, nineteen years her senior and the most powerful regent and mayor of Amsterdam, and thus became "first lady of Soestdijk", then one of their country houses.
Split into simpler sentences: At the age of sixteen she married Cornelis de Graeff, nineteen years her senior and the most powerful regent and mayor of Amsterdam. Thus she became "first lady of Soestdijk", one of the family's country houses.