They both had film star good looks, were adored by the public and their love lives captivated their millions of fans.
Princess Diana and Princess Margaret had so much in common.
But the two women were reportedly at the centre of one of the most toxic feuds in the royal family - and it all boils down to one interview.
Their relationship is explored in new Channel 5 documentary, The Royal Family At War.
Princess Diana's former butler, Paul Burrell said: "Princess Margaret wasn't a fan. She was a film star and the Diana of her day and Diana came along and usurped her."
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But things are said to have reached boiling point following Princess Diana's 1995 interview where she discussed the inner workings of her marriage to Prince Charles.
Royal experts say Margaret took this as the "ultimate betrayal" and never spoke to Diana again.
The Queen's former press secretary, Dicker Arbiter, said: "She would turn any magazine that had Diana on the cover over. It was a cold war."
The siutation was made even more uncomfortable because the two royal princesses both lived at Kensington Palace.
Margaret did everythng she could to avoid Diana but according to experts that didn't mean she wasn't aware of almost all of her movements.
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In an attempt to be discrete about her love life, Diana was said the sneak men into her apartment in Kensington in the trunk of a car
Paul Burrell said: "You could see the curtain twitching and Margaret would be watching.
"Diana knew this was happening as well and would wave back."
Another royal expert syas she believes Margaret disliked Diana because she took the limelight away from the Queen.
Journalist Victoria Arbiter said: "Margaret could really hold a grudge and she wanted people to behave as she believed a royal should.
"She was quite grand and didn't identify with Diana. She thought it should be all about the Queen.
"She supported her sister until the end of her life."
- The Royal Family At War is on Channel 5 at 10pm tonight.
