Mr Warwick, who knew the late Countess of Snowdon - as Princess Margaret was also known - personally, said he once asked how she felt about the different education she and Queen Elizabeth received.
"Yes, that was always a bone of contention," the Princess replied.
Previous historical accounts of the Windsor family, including Netflix series The Crown, claimed the Queen's objection to Margaret's marriage to Group Captain Peter Townsend caused a lifelong rift between the siblings.
As head of the Commonwealth and the Church of England, Queen Elizabeth was forced to bow to pressure from the British parliament and refuse her sister's wish to marry Peter Townsend because he had been married before.
Royal author Katie Nicholl has said the issue caused Princess Margaret to resent the Queen for the rest of her life, leading to "difficulties and tension" within the royal family, particularly after Elizabeth was allowed to marry her undeniably controversial love Prince Philip of Greece, now Duke of Edinburgh.
Princess Margaret went on to marry society photographer Anthony Armstrong-Jones at London's Westminster Abbey in 1960.
The royal couple welcomed two children, Lady Sarah Chatto and David Armstrong-Jones, Earl of Snowdon, but the marriage ended in divorce in 1978.
It was the first divorce of a senior member of the British royal family since Princess Victoria Melita of Edinburgh's in 1901.
Princess Margaret passed away aged 71 in February 2002, seven weeks before the Queen Mother's death.
Since Princess Margaret's death, sweeping changes have taken place within the royal family.
Prince Charles and Princess Diana's eldest son Prince William, the future King of England, married his common-born college sweetheart Kate Middleton in a fairytale royal wedding at Westminster Abbey in 2011.
The Duke and Duchess of Sussex welcomed the first biracial royal baby, Archie Harrison Mountbatten Windsor, on May 6 2019.