A royal expert is speaking out this week to say that Princess Margaret, the late sister of Queen Elizabeth, struggled with being a “spare” throughout her life.

Royal author Andrew Morton just wrote the book “Elizabeth & Margaret: The Intimate World of the Windsor Sisters,” which explores the Queen and Margaret’s fierce devotion to each other. For the book, Morton talked to many of the closest confidantes to Margaret, who died in 2002.

“So much focus has been on Princes William and Harry,” Morton told Fox News. “However, I wanted to explore the lives of these sisters who had a unique relationship that forever impacted the monarchy. They were incredibly close, yet like salt and pepper. To understand the queen, you need to understand Margaret, which gives a greater picture of the Windsors and their reign. It was a fascinating project, one that gave me a completely different perspective.”

“Margaret was faced with struggles as a spare,” he added. “Every spare faces it, not just Margaret. The challenge is to find a relevant and meaningful role in life. That way you are not seen as someone standing in the shadow of somebody else, someone who is going to be king or queen. It’s difficult. And as you go down the line of succession, you become less relevant to the monarchy. There’s a pecking order and you’re suddenly at the bottom.”

Tragedy struck in 1952, when their father died at age 56 from coronary thrombosis, which is a blocking of blood flow to the heart as a result of a blood clot in an artery.

“Margaret adored her father,” said Morton. “Just trying to keep it together on a daily basis was a challenge. She was drinking too much. She was smoking too much. People were very concerned about her mental and physical health. As for the queen, as she said, she had a job. She was grieving, but she had a family, she had a support system behind her. And she had to take on this enormous role, which is becoming queen. The crown had to come first. But for Margaret, she was just absolutely devastated.”

Despite their different personalities, Margaret and the Queen remained close over the years.

“In our family, we don’t have rifts,” Margaret once said. “We have a jolly good row and then it’s all over. And I’ve only twice ever had a row with my sister.”

Margaret died “peacefully” in 2002, according to Buckingham Palace.

“I hope readers of my book will get a sense of who these women are,” said Morton. “It’s a story that anyone with a sibling can identify with. It’s about understanding the complex dance between two sisters with different priorities, different paths. But in the end, they supported each other.”