Andrew Morton, the author of the bestseller "Diana: Her True Story" whose primary source was Princess Diana herself, has written about her relations with Princess Margaret in his new book.

In the biography titled "Elizabeth & Margaret: The Intimate World of the Windsor Sisters," Morton writes that Princess Margaret sympathised with Diana, the eldest daughter-in-law of her sister Queen Elizabeth II. When Diana married Prince Charles, Margaret welcomed her as a breath of fresh air even though other members of the royal family had doubts about her.

The Queen's only sibling saw a kindred spirit in the Princess of Wales, who was struggling to find her own place within the royal family. Morton tells People magazine about the equation between the two late royals: "Neither of them were hunting, shooting or fishing types. Margaret put her arm around Diana. She could see they were two metropolitan princesses."

To ease Diana into the royal family, Margaret took her shopping, to theater, brought her along on social events, and showed her the ropes at the palace. She even suggested for her sister to cut Diana some slack as she was having "difficulties adjusting to her role" as the future Queen consort. Morton writes in his book: "The Queen, who avoids family confrontation if at all possible, took her sister's advice."

On the other hand, Diana also appreciated the help of her "royal guardian angel." She reportedly told Morton about Margaret: "I've always adored Margo. I love her to bits, and she has been wonderful to me from day one."

However, Margaret's opinion of Diana changed after the latter's infamous interview with BBC Panorama, where she spoke of her "crowded" marriage with Prince Charles among other things about the British monarchy. It was then that "the guillotine really came down."

Margaret, who passed away in 2002, five years after Diana lost her life in a car crash in Paris, "had always defended the Queen and was always loyal to her." Morton says: "She felt Diana let her sister down. She was the Praetorian guard of her sister." After the interview, Margaret sent Diana a "wounding and excoriating" letter writing that she had been "incapable of making even the smallest sacrifice" for the monarchy. She was so "angry and upset" that she threw out every magazine that had Diana's picture on the cover, and went on to burn all their correspondence.

Margaret cut off all her contact with Diana and asked her children not to "fraternise with the enemy," while the latter was left "devastated" by her hostility.

the Queen at 90
4 August 1990: A floral fantasy for the Queen Mother's 90th birthday. The Queen, Princess Diana and Princess Margaret all dressed in vibrant, playful designs for the celebrations. Getty