"John had measured most of the women in his life by Jackie's opinion of them," writes J. Randy Taraborrelli in The Kennedy Heirs

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One month after the death of his mom Jacqueline Kennedy Onassis in May 1994, John F. Kennedy Jr. told a friend he was ending his relationship with girlfriend Daryl Hannah. There was, he said, a new woman in his life, Carolyn Bessette.

According to a new book excerpted in this week’s PEOPLE, J. Randy Taraborrelli’s The Kennedy Heirs — which explores the lives of the family’s third generation — his mom had never fully approved of John’s relationship with the actress.

“John had measured most of the women in his life by Jackie’s opinion of them,” writes Taraborrelli. “If his mother approved, John would decide he wasn’t sure. However, if his mother disapproved, as she did with actress Daryl Hannah, John became even more attached.”

Still, he always cared about her opinion.

Says Taraborrelli, “It had always been tough for his girlfriends to meet Jackie for the first time.”

Kennedy Family
Daryl Hannah and John F. Kennedy Jr.
| Credit: Brooks Kraft LLC/Sygma/Getty

John’s mother never met Bessette, the gorgeous fashion publicist whom he married on Sept. 21, 1996. After Jackie’s death, Bessette would meet the matriarch of the family, Ethel Kennedy.

After Carolyn’s first visit to the family compound in Hyannis Port, Massachusetts, she didn’t feel like she fit in and vowed not to go back.

John F. Kennedy, Jr. and his wife Carolyn Bessette Kennedy
John F. Kennedy Jr. and Carolyn Bessette Kennedy
| Credit: Arnaldo Magnani/Liaison/Getty

But, according to the new book, after John and Carolyn had a big fight in Central Park that was caught on film in February 1996, Ethel summoned her to her home. There, she told Carolyn to always keep her emotions in check when in public. According to Taraborrelli, she said, “Don’t let these men goad you into bad behavior. You have to be better than that and you have to be bigger than that.”

“It was tough love because she wanted Carolyn to make it,” says Taraborrelli, “and she sensed her vulnerability.”

RELATED VIDEO: Story Behind The Story: John F. Kennedy Jr.

Taraborrelli also writes that John’s sister Caroline took a harsh view of Carolyn after the public blow up.

“Caroline was critical of Carolyn and felt she should have known better,” says Taraborrelli.

John, however, was always reluctant to disagree with his sister. “Caroline never approved of any of the women in his life, possibly with the exception of the actress Christina Haag, whom he dated before Daryl,” says Taraborrelli. “He had come to the conclusion that she’d never be happy with anyone he chose. He would just listen and then do what he wanted to do.”

“Caroline is a resilient strong woman who feels like you need to take care of yourself and she didn’t feel like Carolyn had what it took to be a Kennedy wife,” adds Taraborrelli. “She was very protective of John and that prevented her from extending herself when Carolyn needed help.”

Kennedy Family
John F. Kennedy Jr. and Jacqueline Kennedy Onassis
| Credit: Time Life Pictures/DMI/The LIFE Picture Collection/Getty

“Both Caroline and Jackie were tough on John’s girlfriends because they wanted someone for him who would make them feel like he was being taken care of,” says Taraborrelli.

“John always measured the women in his life by the opinions of his mom,” adds the author. “After she was gone, John had to make up his own mind. He went full throttle with Carolyn, which shows a lot of growth and maturity on his part.”

For more about JFK Jr. and Carolyn Bessette, pick up the latest issue of PEOPLE, on newsstands Friday.