Willie K’s sister, the Duchess of Marborough
Approximately 5,000 people gathered at the Mineola grandstand including William K. Vanderbilt Jr.’s sister, Consuelo, the Duchess of Marlborough (center wearing a white blouse). Encouraged by her mother, Alva Vanderbilt, Consuelo married England’s Duke of Marlborough in 1895.
The link above didn’t work. Here’s the map