John McCain's First Wife Opens Up About Him Leaving Her to Marry Cindy: 'It Broke My Heart'

It's been almost 40 years since Sen. John McCain left his first wife and three young children to marry Cindy Lou Hensley, a woman 18 years his junior — yet a lingering pain remains evident on the face of his oldest daughter, Sidney McCain.

"We were all shocked and heartbroken," Sidney Sidney said on the John McCain: For Whom the Bell Tolls, a documentary that premiered Monday on HBO and details the extraordinary life of the Arizona Republican currently battling stage 4 brain cancer. "It caused quite a rift within the family."

Sidney, 51, and her brothers Doug, 58, and Andy, 56, didn't attend their father's second wedding in 1980, although over the years they grew close to their father's new wife, PEOPLE reported in 2008.

In the documentary, the 81-year-old McCain's three oldest children as well as his ex-wife, Carol, 81, open up about the wounds of those difficult early years to filmmakers Peter, Teddy and George Kunhardt.

"Cindy was very young too, and you can't help who you fall in love with," says Sidney. "I truly believe that my dad is very much in love with Cindy, and I think she is very much in love with him, and I think there is something very beautiful about that. At the time it was really awful."

When McCain married Carol Shepp in 1965 he adopted her sons, Doug and Andy, whom she had during her first marriage. John and Carol then had a daughter, Sidney, in the fall of 1966. A year later, McCain was captured as a prisoner of war in Vietnam and held for five-and-a-half years.

Upon McCain's return, he became a commanding officer in a Naval squadron, followed by a job as a military liaison for the Senate. "We traveled all over the world together," former Vice President Joe Biden says in the film of those days with McCain.

Andy recalls: "As dad progressed, he was gone a lot, doing a lot of international travel, and it was putting a little stress on the marriage."

During this time in 1979, McCain met Cindy Hensley in Hawaii.

"I was a teacher of special education at the time in Arizona, and it was spring break and we were invited to a reception that was being held for a group of United States senators on their way through Hawaii to China," Cindy, 64, recalls.

"He introduced himself to me and I just didn't know what to expect. What I saw was this incredible human being that was a lot of fun to be around," she continues, as viewers see a photo of a young and smiling Cindy sitting next to a brimming McCain.

Within a year of meeting Cindy, the still-married McCain, then 43, asked the 25-year-old Cindy to marry him.

"I really didn't think he'd propose," Cindy says. "He was older. I knew he cared very deeply for me. I did know that."

According to the Washington Post, John and Cindy obtained a marriage license in Arizona in early March 1980, four weeks before his divorce from Carol was final. They married six weeks later, on May 17.

In one of her few public comments about the marriage, Carol tells the filmmakers that her then-husband's introduction to Cindy "was about the time our marriage was falling apart."

"He was looking for a way to be young again, and that was the end of that," she also says. "I didn't know anything about it, I had no idea what was going on, I was pretty much blindsided and it broke my heart."

Doug McCain recalls that the breakup "left a bad taste in my mouth 'cause I knew it wasn't what my mother wanted. By the same token, you know that sometimes things are beyond your control. I think the divorce rates among the POWs were extraordinarily high so in hindsight, it's probably not unexpected."

While the initial shock of the divorce appears to remain painful, there has been significant healing.

"A lot of people tried to get me to say bad things about him during that time," Carol tells the filmmakers of the 2008 presidential campaign. "And I was like, 'Are you crazy? I would never do that, you don't know me or you wouldn't ask me.'

"I mean, I love the man. I would never do anything to harm him in any way," she says. "I am very sad that he is going to be leaving us in the next year. It's heartbreaking. It's not fair."

Meanwhile, daughter Sidney and second wife Cindy have reportedly developed a relationship over the years. Andy McCain has been a longtime executive for Cindy McCain's beer distribution business, Hensley Beverage Company.

These days, all three kids from McCain's first and second marriage agree that it's been Cindy who "has been a rock" during the senator's struggle with glioblastoma, a very aggressive form of brain cancer, says filmmaker Peter Kunhardt.

She's "been the point person to keep everybody informed and she's been there for John," Kunhardt tells PEOPLE. "She continues to be a very strong influence."