Complex sentence: Elen was mother of five, including a boy named Cystennin (or Custennin or Constantine), but she lived about sixty years later than Helena of Constantinople, the mother of Constantine the Great with whom she has, in times past, been confused.
Split into simpler sentences: Elen was mother of five, including a boy named Cystennin (or Custennin or Constantine). She lived about sixty years later than Helena of Constantinople, the mother of Constantine the Great with whom she has, in times past, been confused.