Princess Margaret Saw a Kindred Spirit in Princess Diana Until 'the Guillotine Came Down,' Says Andrew Morton

When Princess Diana joined the royal family, Queen Elizabeth's sister, Princess Margaret, welcomed her as a breath of fresh air amid others' doubts.

At first, Margaret saw a kindred spirit in Diana, who struggled to find her own place within the royal family. 

"Neither of them were hunting, shooting or fishing types," Andrew Morton tells PEOPLE in this week's issue. "Margaret put her arm around Diana. She could see they were two metropolitan princesses." 

"Princess Margaret came to her rescue, suggesting to the Queen that Diana was having difficulties adjusting to her role and that she should cut her some slack," Morton writes in his new book Elizabeth & Margaret: The Intimate World of the Windsor Sisters. "The Queen, who avoids family confrontation if at all possible, took her sister's advice."

Morton tells PEOPLE of Margaret, "She did say, 'Just give Diana a chance."

Can't get enough of PEOPLE's Royals coverage? Sign up for our free Royals newsletter to get the latest updates on Kate Middleton, Meghan Markle and more!

But in 1995, after Diana aired her grievances to the BBC's Martin Bashir in her explosive Panorama interview, "the guillotine really came down," says Morton. 

RELATED: The Biggest Bombshells From Princess Diana's Panorama Interview in 1995

Breaking away from the royal family's practice of keeping a "stiff upper lip," Diana spoke candidly with Bashir about the breakdown of her marriage to Prince Charles, the pressures of royal life, her mental health struggles and the lack of support she felt within the royal family.

Get the premiere issue of PEOPLE Royals for glamorous new photos and inside stories royals fans haven't seen or read elsewhere! Subscribe at peopleroyals.com/launch

Margaret, who died in 2002 at age 71, "had always defended the Queen and was always loyal to her. She felt Diana let her sister down. She was the Praetorian guard of her sister."