After Michelle “Shelly” Knotek beat and tortured her first murder victim, Kathy Loreno, to death in 1994, she gathered her husband, nephew and daughters.

“All of us will be in jail if anyone finds out what happened to Kathy,” she warned.

The threat resonated with her oldest, Nikki, now 44, who is interviewed with her two sisters in the new book “If You Tell: A True Story of Murder, Family Secrets, and the Unbreakable Bond of Sisterhood” (Thomas & Mercer).

In it, Nikki reveals to author Gregg Olsen how the family’s enforced code of silence led to the deaths of two others: Shelly’s nephew, Shane Watson, 19, and her 57-year-old friend, Ron Woodworth. “They were innocent people who got caught in Shelly’s evil web,” Olsen told The Post.

Shelly’s daughters reached out to Olsen in 2017 and asked him to write the book about their mother for one simple reason: They want to stop her from killing again.

The now-65-year-old is due to be released from the Washington Corrections Center for Women in June 2022 after serving 18 years for second-degree murder and manslaughter. She took a plea deal but did not admit guilt.

Although the case received much attention when the crimes were uncovered in 2003, Shelly’s children — Nikki, Sami, 41, and Tori, 30 — want to remind people what a monster their mother is and to warn about her release.

Shelly’s reign of terror took place in the rural town of Raymond, Wash.

According to the book, she emotionally and physically abused her daughters, who all had different fathers. As well as beating them, her favorite punishment was something she called “wallowing”: Shelly would force them to strip naked in the middle of the night and roll in mud while she hosed them with cold water.

Such retribution was imposed for perceived slights such as visiting the bathroom without asking.

The book recounts how she made the girls sleep outside and how she once shoved Nikki head-first through a glass door. “Look what you made me do,” was Shelly’s response, per “If You Tell,” as blood dripped from her daughter’s face and body.

Olsen writes about how she attacked Sami so often that the girl wore pants to cover her bruises, lest teachers ask anything.

When Shelly’s teenage nephew, Shane, came to live with the family in 1988, she’d pressure him and 13-year-old Nikki to slow-dance in the living room while nude, per the book.

No one really knows why Shelly — who, according to “If You Tell,” also lied that she had cancer and even shaved off her eyebrows to fake the effects of her “treatment” — committed her atrocities. Olsen describes her in the book as “Cujo, Freddy Krueger . . . Pennywise.”

Other times, she would, confusingly, shower the children with affection.

“It worked like any abusive relationship,” Nikki recalls in the book, “and then the abuser reins them back in with kindness.

“My mother was like a ticking time bomb. I never knew when she would go off.”

The cruelty was kept within the family until the December 1988 arrival of Kathy Loreno, an old friend of Shelly’s. The 30-year-old woman had fallen out with her family and moved in with Shelly, agreeing to babysit in exchange for free room and board.

Employing her usual mental manipulation, Shelly initially love-bombed her friend before degrading her, drugging her with tranquilizers and withholding food.

“Kathy was a pleaser and never did anything to trigger such treatment,” said Olsen. “Shelly delighted in making other people hurt. It made her feel superior. She has never been formally diagnosed as a psychopath, but showed all the traits.”

She accused Kathy of binge-eating while sleep-walking. She forced her to sleep in the boiler room, took her clothes and made her work naked.

Once, Sami was getting off the school bus when her classmates spotted a nude Kathy outside the isolated home.

Shelly covered her tracks the next day by inviting over one of Sami’s friends and saying it was her — and that she had jumped out of a hot tub after it “short-circuited.” She showed the girl a hole where the wires had supposedly caught fire, but which she had deliberately burned into the tub.

Incredibly, Kathy stayed put for five years because she was so terrified of the consequences of trying to escape. At a certain point, she couldn’t: Shelly and her husband, Dave, tortured the woman using makeshift waterboarding equipment. Other times, they duct-taped her arms and legs together and poured bleach into the open sores that covered her body.

The children were aware of the abuse and live with the regret of not intervening — but admit there was little they could do. Besides, the abuse took the heat off them.

“[If] Mom was punishing Kathy, she was ignoring us,” Nikki said. “As sick as that was . . . we were glad Mom wasn’t doing it to us.”

Kathy lost more than 100 pounds before she died in July 1994, while imprisoned in the laundry room.

Her family, having no idea of the truth, posted a missing-persons ad in a local paper. Dave later claimed she had choked on her own vomit, but they hadn’t taken her to a hospital or reported her demise because her injuries would implicate them.

Instead, Shelly and Dave burned the corpse in their backyard and disposed of the ashes in the Pacific Ocean. None of her remains were ever found.

The next person to vanish was Shane, the nephew who had moved in with the family while his father, Shelly’s brother, was in jail.

In February 1995, Shelly became convinced the 19-year-old was going to rat them out after Nikki had told her that he had photos documenting Kathy’s abuse. (The photos were never found.)

She persuaded Dave to shoot Shane in a shed on the property, again burning the body and throwing the ashes into the ocean.

“To this day, Nikki wonders whether she is partially to blame for Shane’s death,” said Olsen, adding that the climate in the home was “at boiling point” in the months following Kathy’s murder.

Shelly had a cover story for Shane’s disappearance, telling her daughters — who adored their cousin — that he had run off to become a fisherman in Alaska. When anyone asked about him, she’d say he had recently called her and was happy in his new life.

“Nikki, Sami and Tori wanted it to be true,” said Olsen.

They did have doubts, however, as Shelly claimed that, before Shane left, he had presented her with a wooden birdhouse and a note saying he loved his aunt.

Nikki recalls thinking that it didn’t add up because Shane hated Shelly, who once bound his wrists and ankles with duct tape and applied Icy Hot cream to his penis.

Shelly’s third victim, Ron Woodworth, 57, was another friend who boarded with the family in 1999. By then, Sami and Nikki had moved out, leaving just Tori at the home. Dave, a logger and contactor, was working on an island 160 miles from Raymond.

Like Kathy, Ron was at first showered with a warm welcome. But soon he was so dehumanized by Shelly — who confiscated his clothes and labeled him “useless” — that he gave in to her demands to drink his own urine. She also made him jump off the roof of the two-storey home, smashing his feet into the gravel.

His wounds were “treated” with boiling water and bleach. As Tori told Olsen: “It was like the smell of bleach and decomposing flesh, like it was burning his skin off . . . He smelled like that for a month. Up until the very end.”

Ron died of his wounds in August 2003. Shelly hid his body in her freezer for four days, until Dave returned and buried Ron in the backyard — intending to set the body on fire once a local burn ban had been lifted.

That same week, the sisters ­reunited in Seattle, where Nikki now lived. Tori had had enough of her mother’s abuse. “She needs to be stopped,” she told her sisters.

They called police. Tori, then 14, was removed by Child Protective Services while the property was searched. After Ron’s body was found, Shelly was charged with two counts of first-degree murder in the deaths of Ron and Kathy. Dave was charged with the first-degree murder of Shane.

The couple both pleaded guilty to lesser ­charges in 2004.

Shelly was sentenced to 22 years in prison but is scheduled to be released in 2022 for good behavior. Dave, now 66, was paroled in 2018 after serving 15 years. He has kept in touch with Sami and his biological daughter Tori, who have said they forgive him. Nikki, however, won’t talk to him because of his role in her personal abuse.

Mercifully, the three women have been able to move on with their lives. Nikki works in her husband’s landscaping business in the Seattle area, Sami is a teacher in Raymond, and Tori has a social-media job in Colorado. Sami and Nikki are married and have three children each.

“It’s testimony to their strength and resilience that they are so successful and happy,” Olsen said, explaining that it’s all the more reason for them to fear their mother’s re-entry into society.

The trio last spoke to their mother when she was arrested, although Sami corresponded with her when she was first jailed. “Shelly would ask her to send expensive lingerie and Sami realized she was using her,” Olsen said.

The daughters say they don’t have concerns for their own safety once their mother is freed. But they worry about others.

Olsen said of the three women: “They feel it’s their duty to expose their mother’s true nature as a predator.”