Complex sentence: He was the youngest child and only son of John Henry Hammond and Emily Vanderbilt Sloane, one of three daughters of William Douglas Sloane and Emily Thorn Vanderbilt and a grand-daughter of William Henry Vanderbilt.
Split into simpler sentences: He was the youngest child and only son of John Henry Hammond and Emily Vanderbilt Sloane. His mother was one of three daughters of William Douglas Sloane and Emily Thorn Vanderbilt, and a grand-daughter of William Henry Vanderbilt.