Complex sentence: A biographer of her father, however, makes it clear that this could not have been true: at the time of her son's death, Edith was with him at the family estate, Kykuit, at Pocantico Hills, New York.
Split into simpler sentences: A biographer of her father, however, makes it clear that this could not have been true. At the time of her son's death, Edith was with him at the family estate, Kykuit, at Pocantico Hills, New York.