The niece of Lori Vallow, the Idaho mom who was arrested last week, months after two of her children vanished, knows where the children are, the niece's ex-husband alleges in court documents related to the couple's custody fight over their own children.

Brandon Boudreaux says in the petition filed in Arizona that he has "serious concerns for his children's safety if they were placed in the care of mother" in part because Melani Boudreaux's "knowledge of the whereabouts of her aunt's two missing children and her unwillingness to cooperate with law enforcement in finding those children is daunting to father."

Boudreaux says his ex-wife is also "heavily" involved in a doomsday cult in which "numerous members, adults and children alike, have been being killed off like flies." Vallow has also been accused of being consumed with doomsday theories.

Vallow's 7-year-old son Joshua, who was adopted and has special needs, was last seen alive on Sept. 23, 2019, according to a probable cause affidavit filed against Vallow last week. Her daughter, 17-year-old Tylee Ryan, has not been seen or heard from since Sept. 8, 2019.

Joshua Vallow and Tylee Ryan. Freemont County Sheriff's Office

In early December, after police in Idaho began probing the kids' disappearance, Vallow and her fifth husband, Chad Daybell, flew to Hawaii without any children.

Vallow was arrested in Hawaii on Feb. 20 and charged with two counts of desertion and nonsupport of dependent children, resisting an officer, solicitation and contempt. The next day, she waived her right to an extradition hearing, and is due back in court on March 2.

Before Rexburg police in Idaho went to Vallow's home to check on Joshua, at the urging of his biological grandmother, at the end of November, they were already familiar with Vallow.

A detective had begun monitoring her on Nov. 1 after police in Gilbert, Arizona, told them that a vehicle in her possession was used in the alleged attempted murder of Brandon Boudreaux in October 2019.

"Brandon witnessed the Jeep" and "observed a rifle with a silencer come out of the rear window of the Jeep and then Brandon's vehicle was shot once in the front driver’s doorframe," according to Vallow's probable cause affidavit.

In the custody-related court filings, Boudreaux alleges that his ex-wife plotted with her uncle to carry out the murder.

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Melani Boudreaux's new husband "has even provided statements to law enforcement that mother told him shortly after their marriage that she conspired with her uncle to kill father," the documents say, and he "has even given up his own parenting time out of fear of what the mother may do or have done to his children."

The uncle is not named, but the documents say that he suffered a "suspicious death" in the months following the attempted murder of Brandon. Vallow's brother, Alex Cox, died in December, the Associated Press reports.

Melani Boudreaux is under investigation regarding the attempted murder and the disappearance of her aunt Lori Vallow's children, the documents say. Melani Boudreaux is now seeking custody of her children "as she feels the suspicious death of her uncle who was the shooter in the incident ... has somehow freed her from her role in the attempt on [Brandon's] life."

Brandon Boudreaux says his ex-wife had "a million dollars of reasons" to have him killed as the two had not reached an agreement over their finances yet, and Melani Bordeaux "was hoping to cash in on that policy to support the cult that she is believed to the a part of."

The custody documents say Melani Boudreaux "is heavily involved in a doomsday cult located primarily out of the town in which [she] currently resides" and abandoned her children "in the dark of night" to move to.

Many members of this cult, both children and adults, have been "killed off like flies," the documents said. Melani Boudreaux sometimes says "children are full of light and then just like that they go dark," her current husband has told police, according to the documents.

Melani Boudreaux's attorney did not immediately respond to a request for comment regarding the allegations her ex-husband made in the custody filings.

Vallow has also been accused of being involved in cult-like beliefs. Tylee's aunt, Annie Cushing, told NBC affiliate KSL earlier this month that when she visited Vallow and Tylee in 2018, she noticed that Vallow, who had always been religious, had become obsessed with end-of-times theories.

Lori Vallow was arrested on Feb. 20, 2020 in Hawaii and was being held on $5 million bail, Kauai police said. via Hawaii News Now

“There was one time where she was talking about it and she says, 'Sometimes, I think it would be better just to put my kids in a car and go off the side of a cliff,'" Cushing said.

Cushing's brother, Joseph Ryan, was Vallow's third husband and Tylee's father. He died of an apparent heart attack in 2018, years after he and Vallow had divorced in 2004. An online obituary said Ryan was 59.

Ryan, in court documents related to a custody battle between him and Vallow, said Vallow was ordered to undergo a psychological evaluation after she told social workers that “death would be an option before giving Tylee to her father, Mr. Joseph Ryan, even for a visit.”

Vallow's fourth husband, Charles Vallow, was fatally shot in Chandler, Arizona, by Cox in July, 2019.

Charles Vallow was 62, according to an obituary. The Maricopa County Medical Examiner in Arizona ruled his death a homicide, and police are still investigating. "Our case is still open," Chandler police Detective Seth Tyler told NBC News on Tuesday.

Vallow and Cox were questioned by police but not charged. Cox died five months later.

Before Charles Vallow died, he had filed for divorce, claiming in divorce documents that his wife believed she was reincarnated and was a god sent to lead people during the second coming of Jesus in July 2020 and that she told him that if he got in his way, she would kill him.

Family members told NBC affiliate KSL in Salt Lake City, Utah, that Lori Vallow and her new husband, Chad Daybell, had bonded over their religious beliefs.

Lori Vallow and Chad Daybell. Freemont County Sheriff's office

Daybell, a member of The Church of Jesus Christ of Latter-day Saints, is a self-published author who has written more than two dozen books about near-death and doomsday events through his "Spring Creek Book Company," according to his website. He has not been charged or arrested in connection with his missing stepchildren.

Daybell also spoke several times at Preparing a People, which calls itself a "series of lecture events focusing on self-reliance and personal preparation." Preparing a People is based in both Rexburg, Idaho, and Utah.

Many members of The Church of Jesus Christ of Latter-day Saints are affiliated with Preparing a People, according to KSL, but the founders of Preparing a People says "it is not a 'group' and is not a 'cult.'

In a December statement, the founders, Michael and Nancy James, said they "were shocked by the reports about Mr. Daybell and the missing children," and were cooperating with law enforcement. The statement said Preparing a People had stopped promoting recordings of Daybell's talks, along with podcasts that he and Vallow recorded in affiliation with the group.

Daybell used to host podcasts with his first wife, Tammy, according to his website. Tammy Daybell, 49, was found dead in her home in October, weeks before Chad Daybell and Lori Vallow married.