Ferguson was recently caught trying to sell access to her former husband, Prince Andrew, for nearly $1 million, The Mail Online reported.
Ferguson's invitation was at the insistence of William, royal insiders said. She has been unwelcome at royal family gatherings since her divorce from Andrew in 1996.
Ferguson was also a good friend of William's mother, Princess Diana, although they had a falling out before her death in an automobile accident.
The duchess, 51, will accompany her daughters, Princesses Beatrice and Eugenie to the April 29 wedding. The princesses are close to their cousins, William and Prince Harry.
Ferguson reportedly received more than two dozen offers from American television networks to discuss the wedding, but has so far refused to do so, the report said.
Royal insiders said Ferguson will not be invited to a dinner and dance hosted by Prince Charles at Buckingham Palace following the wedding ceremony.
Ferguson's planned show for the Oprah Winfrey Network called "Finding Sarah," a documentary about her financial recovery, has been pushed back until after the wedding.