“NCIS” actor Mark Harmon’s sister, actress and painter Kristin Harmon Nelson, died on April 27 at age 72 from a sudden heart attack, leaving behind a relationship that was at times turbulent between the siblings.

The star’s daughter, Tracy Nelson, confirmed her mother’s death in a May 1 Facebook post.

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Nelson’s life forever changed when she was merely 12 years old. That’s when she met Ricky Nelson, son of 1950s' icons Ozzie and Harriet.

The family starred in their own sitcom “The Adventures of Ozzie and Harriet" from 1952 to 1966.

Kristin Harmon and Ricky Nelson on their wedding day. (AP)

People magazine reported Kristin Harmon Nelson not only fell in love with the all-American family, but she later fell for the teen heartthrob romantically.

The couple married in 1963, when Harmon Nelson was just 18 and, six months later, their first child Tracy was born. As a pre-schooler, Tracy would get a taste of fame when she appeared in 1968’s “Yours, Mine and Ours” opposite Lucille Ball.

The Nelsons would soon welcome three more children, including twin sons Gunnar and Matthew, the future duo of rock band Nelson.

However, the magazine pointed out Nelson’s story was far from happily ever after. She told People in 1987 that the couple became “hippie rock ‘n’ rollers,” leading to their downfall.

“At first we were in it together,” she explained. “I tried to be one of the guys, to fix the marriage by going on the road and being involved in road stuff that is really not good for anyone. After a while, we were totally messed up, both of us.

The Nelson family. (Zuma)

"I got into therapy and so did he for a while, but then he started not showing up. I tried telling my family, 'There’s a drug problem here and we’ve all got to help.' But they totally denied there was anything wrong.”

Accusations of drug and alcohol abuse plagued the marriage. People added that by 1980, Nelson filed for divorce and was temporarily granted custody of the children. However, the divorce wouldn’t be finalized until 1982.

Nelson admitted that during this time, she still used drugs and drank alcohol “quite heavily.” Too drunk to paint for a living, she had to take on a job as an assistant casting director for $200 a week.

Tragedy would then strike in 1985. Ricky, his fiancée Helen Blair and five members of his road company perished in a plane crash while traveling to Texas. He was just 45.

Ricky Nelson passed away in a 1985 plane crash. (AP)

After the horrific crash, Nelson said it was her brother, Mark Harmon, who urged her to get help and enter rehab.

“Nobody but my brother could have talked me into this because I trusted him,” she said. “A voice inside me said, ‘For once in your life, let somebody help you.’”

But in 1987, Nelson learned that Harmon and his wife, actress Pam Dawber, were taking her to court to seek custody of her youngest child, 12-year-old son Sam. Harmon contended that Harmon Nelson was unable to take care of Sam because of her drug dependency.

Harmon Nelson told Star magazine at the time that the custody battle started when her brother persuaded her to check into a clinic for treatment.

“He said, ‘You’ve got to get off those pills and you need a good, long rest,’” she recalled.

Nelson claimed she learned of the custody action on the day she left the hospital.

“I was feeling very proud and happy because I had just very successfully completed a program to help myself,” she explained. “Suddenly, I had two hours to find a lawyer who would help me keep my son. At first, I thought it was a joke, and I wondered, ‘Where’s the punchline?’”

Mark Harmon urged his sister to get help for drug addiction. (Reuters)

People noted that Harmon was awarded temporary custody and Harmon Nelson was ordered to stay 200 yards away from her brother’s home.

But Harmon, tired of the family feud, reportedly reached out to his sister’s attorney and declared, “We don’t want to go any further with this. Too much blood has been spilled.”

The family would ultimately work out an agreement. Harmon Nelson retained custody while her brother was granted visiting rights to ensure his nephew was in a safe household. The trio also agreed to enter family therapy.

In 2016, Sam told Medium his relationship with his mother improved over the years.

Sam Nelson

“As you get older, things that were important aren’t important anymore,” he explained. “Problems aren’t problems anymore. You work through them, or you don’t. But for the most part, you hope to. I think we’re in a good way together, and we have a good relationship.”

The Associated Press contributed to this report.