The Duchess of York said the late Diana, Princess of Wales, would be "very proud" of her son and their "wonderful wife".
In a new interview, Sarah Ferguson told People magazine that Diana would "spoil her grandchildren."
When her daughter Princess Beatrice gives birth, the 61-year-old will become a grandmother for the second time.
Her youngest daughter, Princess Eugenie, gave birth to her first child in August this year.
The Duchess was affectionately known as "Fergie" and had been good friends with Diana since she was a teenager.
The two women know each other well, and Fergie describes their relationship as "best friends".
She was asked what Diana would think of her sons, Prince William and Prince Harry, whose relationships had deteriorated in recent years.
Recently, William and Harry shelved their differences to unveil a statue of their mother in kensington Palace's sunken gardens.
"She would be very proud of her son and their wife," the Duchess of York told People magazine.
She will spoil her grandchildren, just like me, because that's what she loves. ”
Fergie said Diana was "in love" with her daughters Beatrice and Eugenie.
"If she were sitting with me right now, I know she would say, 'I'm very proud of my two sons and the excellent wife they've chosen,'" Fergie said.
"Because everyone has their own voice."
Neither Kate nor Megan had ever met Diana.
Prince Harry had previously said that his mother would like Meghan.
In an engagement interview in November 2017, Prince Harry said: "They're going to be as intimate as thieves, and there's no doubt I think she'll be ecstatic, jumping up and down and very excited for me, but like I said, she could be Megan's best friend." ”
Diana's biographer Andrew Morton wrote Diana: Her True Story in 1992.
Recently he speculated on how the Princess of Wales would see her two daughters-in-law.
Morton said in an article to The Mirror: "She would be close to Catherine, and her initial shyness on the public stage reminded her of her early fears in front of screaming crowds." ”
As for Megan, she would find this energetic American charming and intimidating.
Diana aspired to be famous for her speeches rather than fashion, so she learned how to give speeches.
"When she first met Megan, she might be surprised because Megan was an innate strength to speak passionately and accurately."
But when she got to know Diana and saw the vulnerable side of this woman who suffered trauma during her first pregnancy, they bonded. So was Diana. ”