The 59-year-old Duchess posed for pictures with her half-sisters Alice Stileman and Eliza Cobb, who are also the daughters of Sir Ronald and his second wife Susan Deptford.
Sir Ronald, who was polo manager for Prince Philip, the Duke of Edinburgh, and later for Prince Charles, died following a heart attack in 2003.
Fergie also has a sister called Jane, Daily Mail reports.
Fergie's family wedding comes amid mounting speculation that the flame haired royal will walk down the aisle once more herself, to marry Queen Elizabeth's son for a second time.
The Duke and Duchess of York, who are parents to Princess Beatrice, 30, and Princess Eugenie, 29, have sparked remarriage rumours since late last year when they put on a very cosy show at Eugenie's royal wedding to Jack Brooksbank at St George's Chapel in Windsor Castle last October.
The world's 'happiest divorced couple' have been inseparable of late, attending a number of royal events together including the wedding of Lady Gabriella Windsor and Thomas Kingston last month.
Fergie and Prince Andrew also went on a family holiday to Bahrain with Princess Beatrice earlier this year.
"Andrew is doing everything he can to bring Fergie back into the royal fold," a Palace was quoted as saying in the wake of Eugenie's wedding.
The Duke of York no longer needs the Queen's permission for a royal wedding, since the birth of Prince William and Kate Middleton's third child Prince Louis last April pushed Andrew back to seventh in line to the throne.