Sarah Ferguson said Princess Diana would be "so proud" of son Prince Harry and his wife Meghan Markle and they should not be judged.
The Duchess of York, who is the ex-wife of Prince Andrew, made the remarks during an interview to promote her new fictional book about royal life.
In a nod to the Sussexes' interview with Oprah Winfrey, Fergie said: "There should be no judgement on race, creed, colour or any other denomination."
During the bombshell chat in March, Harry and Meghan claimed there had been "conversations" within the Royal Family about the colour of Archie's skin before he was born.
The Sussexes did not say who made the alleged remarks, but after the interview, Harry told Oprah it was not his grandmother, the Queen, or his grandfather, Prince Philip.
While attending a royal engagement after his brother's interview, Prince William was asked about the claims and he said the royals are "very much not a racist family".
This week, when asked by Good Morning America what she thought of how Meghan had been treated by the press, Fergie said: "I wish Harry and Meghan so much happiness. I know Diana would be so proud of her sons and their wives."
Fergie went on to say she and Diana experienced "tough" times with the British media - and now, in her book 'Her Heart for a Compass: A Novel', she is "finding her voice".
The book is about a "rebel" royal in 1865 whose only desire is to follow her heart.
Harry and Meghan stepped down as senior royals last year and moved to the US, where they have been working on humanitarian projects through their charity Archewell.
The pair opened up on their struggles as royals and the mental health pressures they experienced.
The Duke of Sussex also criticised his dad Prince Charles, and said he and his brother William were "on different paths".
However earlier this month the two brothers put their differences aside and came together for an event to unveil a statue for their mum Diana on what would have been her 60th birthday.
In an emotional joint statement, Harry and William said: "Every day, we wish she were still with us."
This month, Harry also announced he is writing a memoir about life in the royal family.
He has agreed a publication deal with Penguin Random House, which described the book as an "intimate and heartfelt memoir from one of the most fascinating and influential global figures of our time" and promises “an honest and captivating personal portrait.”
In a statement, Penguin also adds Harry will share his "adventures, losses, and life lessons" - including from his time on the frontlines in Afghanistan.
