Complex sentence: He was the eldest son of William Murray, 3rd Earl of Dunmore, and his wife, Catherine (née Murray); he was a nephew of John Murray, second Earl of Dunmore.
Split into simpler sentences: He was the son of William Murray, 3rd Earl of Dunmore, and his wife Catherine (née Murray). John was the eldest son of William and Catherine Murray, and nephew of John Murray, second Earl of Dunmore.