F. Donald Nixon, a younger brother of former President Richard M. Nixon, whose fortunes also became tangled in the web of the Watergate scandal, died in June 1987 at Hoag Memorial Hospital in Whittier, Calif. He was 72 years old. Mr. Nixon testified at the 1974 conspiracy trial of former Attorney General John M. Mitchell and former Commerce Secretary Maurice Stans regarding campaign financing. Both were acquitted. Shortly before he testified Mr. Nixon took early retirement in the spring of 1974 from his position as a vice president of the Marriott Corporation, the hotel and restaurant chain. Over the years, Mr. Nixon tried his hand at various business ventures and investments, not all of which proved successful. But in 1970, J. Willard Marriott, a friend and supporter of President Nixon, hired him as vice president for community and industry relations. In addition to the former President, Mr. Nixon, who was born in Whittier, was survived by their youngest brother, Edward; his wife, Clara Jane Nixon; a daughter, Lawerene Anfinson; two sons, Donald and Rick Nixon, and five grandchildren.