Original: He was the great-grandson of William Stevenson, brother of Vice President Adlai E. Stevenson, making him a second cousin once removed of presidential candidate Adlai E. Stevenson II.
Simplified: His grandfather Adlai E. Stevenson was Vice-president of the United States to Grover Cleveland, and his cousin Adlai E. Stevenson II was Governor of Illinois and unsuccessfully ran for President twice in the 1950s losing to Dwight D. Eisenhower both times.