"By the time the Queen had the two younger children Helen Lightbody [their first Nanny] had gone and Mabel Anderson, her deputy and a much easier character, was in charge," the author explained.
"She and the Queen were good friends and brought the children up together."
Due to the mother-of-four having more contact with her two youngest children, she became notably closer with them.
"As a result the Queen had much more contact with Andrew and Edward than she ever had with Charles and Anne, and is still infinitely closer to them today."
However, the Queen's children do not appear to share a close relationship - due to having different personalities.
Royal commentator Angela Mollard said on New Idea podcast "Royals” that the siblings are "very different, all four of them."
"They all get together and they’re all happy at family gatherings but there’s a lot of question marks over how much they see each other between times."
Mollard explained that during the years where Prince Charles, Prince Andrew, Sarah Ferguson and Princess Diana were at the forefront, things appeared to be "very strange."
"The relationship with Diana and Fergie – initially close – became quite strained.
"They all had a lot going on, they had a lot of broken relationships.
"Three of them having broken marriages, you wonder to what extent they were supports for each other."
The royal commentator was doubtful.