Heiress is one of seven billionaire relatives with a combined net worth of $26 billion thanks to vast wealth created by Cargill, the largest private company in the U.S. The family owns an 88% stake in the company whose 2010 sales were $107.9 billion. Cargill announced in January that it would spin-off its 64%, $24.3 billion stake in Plymouth-based fertilizer maker Mosaic–a move that is said to free up cash and enabling it to remain a private company. William Cargill, son of Scottish sea captain, founded Iowa grain storage business after Civil War. Will’s son-in-law John MacMillan took control 1909. No family member has run the company since Whitney MacMillan stepped down as chief executive in 1995.