Betsey Cushing Roosevelt Whitney, 89, multimillionaire philanthropist and widow of John Hay "Jock" Whitney. Also the first wife of James Roosevelt, Whitney was the last surviving of three Cushing sisters of Boston known for their beauty, charm and influential marriages. Her wealth had been estimated at $700 million. As President Franklin D. Roosevelt's daughter-in-law, she often served as White House hostess when Eleanor Roosevelt was absent. Later, as Mrs. Whitney, she accompanied her husband to England when he was named ambassador to Britain, and befriended both Queen Elizabeth II and the Queen Mother. In 1983, a year after her husband's death, she established the Greentree Foundation, named for her Long Island, N.Y., estate, to fund community groups. She donated money to art, education and medical institutions, including $8 million to the Yale Medical School. Whitney served on several boards, including those of the Whitney Museum of American Art, the John Hay Whitney Foundation and the Assn. for Homemakers Service. Despite her social prominence, she preferred a quiet life managing her estates and charities. On Wednesday in New York.