YOU probably think you know everything there is to know about Kim Kardashian and her siblings.

But there is a little-known prequel — the twisted story of her parents Robert and Kris’ tempestuous marriage, which was marked by rampant philandering and scandal.

Author Jerry Oppenheimer’s book The Kardashians: An American Drama (St. Martin’s Press, out September 19) reveals patriarch Robert’s infatuation with Elvis Presley’s ex-wife Priscilla, Kris’ repeated infidelity and Robert’s confession to his pastor that Khloé Kardashian wasn’t his biological daughter.

HERE, THE HIGHLIGHTS:

Forever immortalised as the stand-by-his-man supporter of pal O.J. Simpson, Robert Kardashian rose to notoriety when he served as a legal consultant on the “dream team” that won the athlete a not-guilty verdict in his infamous 1995 murder trial.

Robert, a born-again Christian of Armenian descent, was born in 1944 to a wealthy Los Angeles family; he would later distance himself from the clan’s corrupt meat-packing empire.

Despite standing a mere 5-foot-8 and being stricken with a thick white hairline streak in his otherwise jet-black mane, he was considered one of Beverly Hills’ most eligible bachelors in the 1970s.

Born in 1955, Kristen “Kris” Houghton came from “redneck” roots and was raised in San Diego by her tough-as-nails maternal grandmother after her alcoholic father left when she was 7. By 12th grade, Kris wasn’t dreaming about prom or college, but looking for a man — a rich one. High-school pal Joan Zimmerman thought Kris’ mum, Mary Jo, was “kind of pimping her out” when the 17-year-old started a relationship with golf pro Cesar Sanudo, who was more than 10 years her senior.

That ended when Kris met Robert — who thought the teenager looked like a young Natalie Wood, despite a necklace that read “OH, S**T” — at a horse-racing track and cheated on her boyfriend with him.

As Jack Spradlin, a friend of Sanudo’s, said: “[Kris] saw a far better financial opportunity with Kardashian than with Cesar.”

Still, Robert thought Kris was too young for things to be serious, and soon dumped her for Priscilla Presley. He may have been besotted with the famous ex-wife of Elvis, but she only went out with Robert because “she had no one else to go out with,” according to a cousin.

He wasn’t lonely for long. A heartsick Kris, by then an American Airlines flight attendant, had been destroyed by Robert’s relationship with glam Priscilla and readily took him back, moving into his Beverly Hills mansion right away.

Where he failed to mould Priscilla into the perfect submissive housewife, Robert was determined to succeed with young Kris.

After daughters Kourtney and Kim were born (in 1979 and 1980, respectively), the family moved into a 7,000-square-foot estate in Beverly Hills’ most “prestigious” section, complete with tennis courts and a duck-shaped swimming pool.

They often hung out with Robert’s friend, O.J. Simpson. Sometimes, Simpson would have Kris call a very young girl — “possibly still in high school” — whom he was seeing, in case her parents answered. Then she would hand him the phone.

It seemed the family was flourishing. Robert had sold one of his businesses, the trade publication Radio & Records, “for a bundle.” But with the 1984 arrival of their third child, Khloé, the couple couldn’t ignore an elephant in the room: They hadn’t had sex during the time she had to have been conceived.

Gulliksen recalled that “it was my strong impression from him that [Robert] loved Khloé very much, but he said it in a way that implied that ‘She’s not my blood daughter.’ ”

He was unwilling to take a DNA test to confirm it, and told Migdal that “whoever her father is ... she is my child.”

Years later, his two subsequent wives — Jan Ashley and Ellen Pierson — attested that Robert claimed that Khloé wasn’t his biological daughter.

Soon, Kris’ affairs would be more out in the open. Despite all the blessings — and bling — of a charmed Beverly Hills life, she became “bored and rebellious.” After getting a new pair of breasts, she reportedly decided she wanted freedom.

The Kardashians finally divorced in 1991. Months later, Kris married Olympian Bruce Jenner, her “best lover.”

Jenner — who decades later would transition to being a woman named Caitlyn — had already wed twice before.

After his first wife, Chrystie Crownover, left him, he stayed for a while at the Playboy Mansion.

“Bruce became like one of the Bunnies,” said a longtime mansion regular. “One night he’s boogieing in a tux with the girls at a dress-up party, and the next night he’d be like one of the girls and all dressed up — makeup, hosiery, high heels, the whole nine yards. I thought he was just being funny, like when Milton Berle used to come on TV in drag.”

He may have been one of the most famous athletes of the 1970s, but by the early ’90s Jenner “had little money ... and was living in a dumpy little house.”

When he moved in with Kris, Jenner brought along his parents.

“Simply put,” Oppenheimer writes, “[Kris] was ‘very pissed off,’ according to friends.”

She would, apparently, get over it, however, with the two going on to have daughters Kendall (in 1995) and Kylie (in 1997) and star together in the reality-TV series Keeping Up With the Kardashians.

Robert, meanwhile, was about to become a household name. After bosom buddy Simpson was arrested in 1994 for the murders of his ex-wife, Nicole, and her friend Ron Goldman, Robert chose to steadfastly support him, showing up to court nearly every day as a member of the athlete’s legal “dream team.”

He wrote a letter to Kris and his kids in 1995, once the trial was in full swing: “I truly believe in O.J.’s innocence, and unless they find him guilty, I will continue to support him ... Please be understanding.”

Kris, who had been one of Nicole’s closest friends, was “furious” with her ex’s defence of O.J. (Years later, Kourtney would get into the University of Arizona with an essay titled “My Parents Were on Opposite Sides of the O.J. Simpson Trial.”)

Elsewhere in Robert’s life, reactions were mixed. Some friends and colleagues deserted him. “No one ever turned their back on someone like they did on Robert over the O.J. thing,” recalled Joni Migdal, one of Robert’s closest friends. People would spit on him while he was in his convertible.

On the other hand, he often got “celebrity treatment” because of the public fascination with the televised trial, snagging the best tables at Beverly Hills restaurants. Roger Moore sought him out to chat. Actor Rod Steiger sent wine to his table. Robert hoped that Al Pacino or Robert De Niro would play him in a future O.J. movie. (Little did he know he would instead end up with David “Ross from Friends ” Schwimmer.)

The fallout from the trial took a toll on the entire Kardashian family. But the kids were moving on with their lives — and the apples didn’t fall far from the tree.

By 2000, Kim was a bride at age 19.

Robert was “upset” when Kim wed her first husband, Damon Thomas, who is African-American. According to an Oppenheimer source, Robert said: “I know these black guys, and I know they love white pussy. O.J. always brags about how much he and those guys get. The problem is my kids are liberal, maybe too liberal, and I have no one to blame but myself because I introduced them to Uncle O.J.”

In 2003, Robert was diagnosed with esophageal cancer; when he died some eight weeks later, he reportedly weighed 36kg. Before he passed, Priscilla Presley called to tell him she loved him.

“It brought tears to his eyes,” Oppenheimer writes.

Years later, after Robert’s kids became world-famous, his widow Ellen Pierson — who was said to have frozen out his closest friends — sold off excerpts of what was allegedly his diary, in which he wrote about Kris and Waterman’s sleeping together in his bed and leaving their kids unattended while she “screwed all night.”

It detailed Kris’ allegedly abusive nature, describing her as pulling Kourtney’s hair and twisting her arms, also claiming that “scared and nervous” Kim had also been beaten by their mother.

Kris sued Pierson in 2013 on the basis of copyright infringement, claiming that Robert’s kids owned the copyrights for his diaries.

Pierson filed legal papers for defamation, emotional distress and civil conspiracy to defame, claiming the Kardashians only filed their lawsuit for use as a plot point for their TV show.

The copyright claim was settled in 2014, when Pierson returned the diaries to the Kardashians, who also collected $A105,000.

Robert’s friend Kraines insists the late man would be as “proud as punch” of his girls today. “Would Robert have liked Kim’s sex tape, and all that horses**t? Probably not. But would he have liked the fact that they have made a tremendous amount of money? Definitely!”

As for Kris, who divorced Jenner in 2015, sources say that she wants to follow in the footsteps of another reality-TV star.

The now-divorced mum of six has told people she wants to run for political office — on a platform of advocacy for single mums — saying she and Donald Trump have the same kind of DNA. “If Mr. Weird Hair can do it, so can I,” she’s reportedly said.

According to a “credible source”: “She has so much confidence that talking about the presidency one day isn’t out of the question for her.”

This article originally appeared on The New York Post and has been republished here with permission.