Complex sentence: Her son William, 3rd Earl of Ulster married Maud of Lancaster, by whom he had a daughter, Elizabeth de Burgh, 4th Countess of Ulster Who became the future wife of Edward III's second son Lionel of Antwerp, 1st Duke of Clarence.
Split into simpler sentences: Her son William, 3rd Earl of Ulster married Maud of Lancaster, by whom he had a daughter, Elizabeth de Burgh, 4th Countess of Ulster. Elizabeth became the future wife of Edward III's second son Lionel of Antwerp, 1st Duke of Clarence.