In the 1997 documentary Fergie: Downfall of a Duchess royal correspondent Judy Wade revealed a “cruelly unfair” moment when the Queen’s sister rebuked Fergie.
The pair had initially been friendly, however their relationship had soured, Ms Wade explained in the YouTube documentary.
Ms Wade commented: “I thought that was the worst example of a pot calling the kettle black in history."
She added: “Princess Margaret was notorious for her behaviour.”
“It was totally out of keeping with her position.”
Margaret had a controversial life herself.
She caused a sensation in the 1950s because of her romance with with Group Captain Peter Townsend.
The princess could not marry the older, divorced Townsend without giving up her royal position – and eventually chose her royal duty over the love of her life.
Throughout her younger decades the princess was also known for her party-girl lifestyle.
Ms Wade continued: “Margaret was cruelly unfair to do such a thing when it was a kind gesture from Sarah.
“I think that indicates exactly how the royal family treats someone who steps out of line – except if they’re born royal.”