When Kaethe Cherney was 19, her older brother, Chris, told her that their mother had tried to kill her as a baby.

“You had a bad attack of croup but she fell asleep [on purpose],” he said. “She wanted you to die.”

The accusation came in 1980. Chris, then 28, had allegedly been brainwashed as a member of The Sullivanians, a psychosexual “cult” based on the Upper West Side.

The controversial group encouraged promiscuity and rejected the idea of the traditional family unit.

“They demonized all moms,” Chris, now 64, told The Post. “They demonized family.”

Then again, the Cherneys were hardly warm and fuzzy. All of that turmoil, plus the effect that The Sullivanians had on Kaethe’s life, inspired her novel, “Happy As Larry: A New York Story of Cults, Crushes and Quaaludes,” (CreateSpace/KDP), out Tuesday.

The main character is a thinly disguised Kaethe, who led a chaotic childhood after the cancer death of her father, Marvin, in 1967 when she was 6.

“That was the beginning of the end [of the family],” said Kaethe, now a 57-year-old writer living in London. “It was the first major fault line.”

The next catastrophe came two years later when the children’s mother, Mimi, took a much younger lover, Danny, who was also Chris and older sister Mary’s music tutor. Complicating matters, Mary had developed a crush on him. She was devastated when her mom “stole” him and appalled when he moved in with them.

“I remember my mother and I sitting on either side of him at the cinema and wondering who he was going to put his arm around,” said Mary, now 62.

The messy dynamic caused resentment, especially when Mimi uprooted the family from their stylish apartment near Gramercy Park to a less desirable building at 106th Street and Riverside.

“It was back when [the Upper West Side] didn’t feel safe,” said Kaethe, who was frequently left alone. “Mom . . . wanted to be my friend more than my mother. I didn’t feel protected.”

Sickened by their mom’s fawning over Danny, who was only a couple of years older than Chris, the older siblings left home when Chris was 19 and Mary was 15. A friend suggested they seek refuge with the Sullivanians at their headquarters on West 98th Street.

Headed by self-styled psychotherapist Saul B. Newton, the organization was founded as The Sullivan Institute for Research in Psychoanalysis, a therapy center, in the late 1950s. Among its members, reportedly, were singer Judy Collins and “Clockers” novelist Richard Price. The group believed that family ties were the root cause of mental illness. “Nuclear families were seen as oppressive,” said Mary.

Polyamory was espoused and so-called “f–k rotas” were displayed in the communal buildings The Sullivanians occupied, telling members who their next sexual partner would be.

“There was a lot of permissiveness,” said Mary.

Chris explained that The Sullivanians appealed to people who came from “broken homes and difficult situations.” He told The Post he couldn’t recall telling Kaethe about their mother trying to kill her as an infant.

Mary stayed with the group for around five years. Chris said he could not remember how long he was a member. Kaethe, who never joined The Sullivanians, claims Chris was in the group for 14 years.

The siblings broke free from The Sullivanians after questioning their beliefs. Around 1977, when she was 21, Mary told a leader that she didn’t want to take part in the group’s occupation of a building in downtown Manhattan. The woman replied: “Well, if you’re not for us, you’re against us.”

With membership dwindling, the alleged cult began to fall apart in the late 1980s after adherents feuded with neighbors on the Upper West Side. Some Sullivanians allegedly beat their neighbors with sticks, but no charges were brought. Newton’s death in 1991 was the death knell for the group.

Today, Chris and Mary are each married and have children, as does Kaethe.

Although it took time, the siblings repaired their relationship with their mom, who died four years ago at 84.

Concluded Kaethe: “Somebody can be complicated . . . but you can still love them.”