The changing relationship between the siblings was especially evident at the Commonwealth Day service at Westminster Abbey in March. "To purposefully snub your sister-in-law ..."
"Also, to remove Meghan from the equation, Harry was exhausted of the dynamic that had become established between him and his older brother, " the authors added. "People just want them to be happy", insisted one well-placed source.
The book also reveals their annoyance at gossipy royal staff who apparently referred to the couple as "the squeaky third wheel" of the palace.
Rather, the authors claim, the alleged rift between the two couples was due to a growing coolness between Harry and William.
But being told to operate under Buckingham Palace's umbrella after splitting their household from William and Kate's was "a big disappointment to them".
Harry and Meghan have been outspoken on the subject of racial injustice, addressing the topic a number of times since George Floyd died during a police arrest in the USA in May.
The authors write: "The Queen made it clear to Harry she would always support him". That didn't happen. "Meghan was disappointed that she and Kate hadn't bonded over the position they shared, but she wasn't losing sleep over it", Scobie and Duand wrote.
The Times newspaper cited extracts from the book suggesting infighting among family members, with Harry believing Meghan was not liked by some of his relatives and others feeling that the popularity of the couple had to be "reined in".
However, in a TV interview with the BBC, the couple say Harry popped the question over a typical night for the pair that included a low-key roast dinner, in November 2017.
The insider told writers Omid Scobie and Carolyn Durand that he could "handle anything after putting up with one of Meghan's temper tantrums".
A new biography of the Duke and Duchess of Sussex has offered a unique insight into what led the couple to step down from royal duties.
A complaint filed at the Los Angeles County Superior Court on Thursday claims an unnamed individual photographed Archie at their home during lockdown.
Ironically, while the more outgoing Harry and Meghan have struggled to find their true identity in the U.S., the Covid-19 crisis has helped the Cambridges to work out exactly who they are - and what they need to do.
They said: "His vision included both his sons".
The move marked the latest example of the Sussexes actions against what they have previously described as "invasive" tabloid media.
The book gives further glimpses of Meghan's troubled relationship with Mr Markle, including a string of unreturned phone messages and texts from the former actress to her father in the days before her wedding.
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