Stacey Lynn Cox Cope died suddenly in 1998 at the age of 31

Lori Vallow, Stacey Lynn Cox Cope
Credit: John Roark/AP/Shutterstock; FindAGrave

Lori Vallow, who is being held on multiple charges stemming from the disappearance of her children, 7-year-old Joshua "J.J." Vallow and 17-year-old Tylee Ryan, has been surrounded by a trail of mysterious deaths -- and a new report uncovers another mystery in her past.

In an exclusive report on Court TV Live on Tuesday night, journalist Ashleigh Banfield uncovered the mysterious death of Vallow's older sister, Stacey Lynn Cox Cope, who died suddenly in 1998 at the age of 31. Her cause of death was undetermined.

"You can look at one death as a coincidence," Banfield tells PEOPLE, "but these people have had a trail of deaths that seemed natural, and are now under investigation. This may be yet another one."

Authorities are currently taking a renewed look at several other deaths surrounding Vallow and her husband, Chad Daybell.

Vallow's fourth husband, Charles, was shot to death during a July 2019 altercation by her brother, Alex Cox, who claimed he was acting in self-defense and was never charged. Three months later, Chad's wife, Tammy, was found dead in her bed -- a death police later called "suspicious." Her body has been exhumed. 

In December, Alex Cox was found dead at his girlfriend's house. Authorities have performed an autopsy and the cause of death is still pending.

In addition, Cox has been alleged as a possible participant in a non-fatal drive-by shooting of Brandon Boudreaux, whose former wife, Melani, is Lori's adult niece -- and the daughter of her dead sister, Stacey Cox Cope.

Robert Jarvis and Garrett Smith, the attorneys for Melani — now Melani Pawlowski — previously issued a joint statement to PEOPLE denying “the innuendoes and allegations … that she has been involved in any wrongdoing.”

“She has cooperated with the FBI thoroughly,” Smith told PEOPLE.

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The children still remain missing, and Vallow is jailed in Madison County, Idaho, on felony counts of desertion and nonsupport of dependent children, as well as resisting or obstructing officers, criminal solicitation to commit a crime, contempt of court and willful disobedience of court process or order for failing to produce the children. She is being held on $1 million bond.

Vallow and Daybell are now officially under investigation for possible murder, attempted murder and conspiracy charges.

In an April 9 letter from the Idaho attorney general's office to the Fremont County Prosecutor's Office, the AG's office agreed to assist local prosecutors in their investigation of those possible charges in Fremont County, where Daybell lived with his wife at the time of her death.

Neither Vallow nor Daybell have been charged in connection with any of the deaths.

Lori Vallow has pleaded not guilty to charges related to the children's disappearance, and is scheduled for a preliminary court hearing May 7 on those charges.

Vallow's attorney, Mark Means, has not responded to any previous calls by PEOPLE for comment. Both the AG's office and the office of Fremont County Prosecutor Marcia Murdoch declined PEOPLE's request for additional comment on the current investigation into possible additional criminal charges.