Complex sentence: His wife was Lady Mary Boyle, daughter of Richard Boyle, 1st Earl of Cork and Catherine Fenton, with whom he became intimate when he helped to nurse her through an attack of measles.
Split into simpler sentences: His wife was Lady Mary Boyle, daughter of Richard Boyle, 1st Earl of Cork, and his wifr Catherine Fenton. He became intimate with Lady Mary when he helped to nurse her through an attack of measles.