"She's not a monster, she's not a heartless, cold-calculated murderous type," Lori Vallow's sister says of the mom of 2 missing kids

Lori Vallow
Lori Vallow
| Credit: John Roark/The Idaho Post-Register via AP

Idaho mom Lori Vallow has her "reasons" for keeping the whereabouts of her two missing kids hidden, her sister says, but those reasons for now remain a secret as well.

"I don't know all of her reasons for doing what she's doing," the sister, Summer Shiflet, says in an interview with 48 Hours to be broadcast Saturday (10 p.m. ET/PT) on CBS. "But I know she has ... has the reasons."

Vallow's two children, 7-year-old Joshua "J.J." Vallow and 17-year-old Tylee Ryan, have not been seen since September, and police in Rexburg, Idaho, have said they “strongly believe that Joshua and Tylee’s lives are in danger."

Vallow remains jailed on a $1 million bond after her February arrest on charges related to her failure to produce the children for police or child welfare workers. She has pleaded not guilty to 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 two kids.

At the same time, authorities are investigating Vallow and her newlywed husband, Chad Daybell, for possible murder, attempted murder and conspiracy charges in the Idaho county where Daybell lived at the time his previous wife, Tammy, died two weeks before he and Vallow were married last November. No charges against either of them have been filed in connection with that investigation.

Vallow's sister and mother, Janis Cox, insist they believe that Vallow has done nothing to hurt the kids.

Want to keep up with the latest crime coverage? Sign up for PEOPLE’s free True Crime newsletter for breaking crime news, ongoing trial coverage and details of intriguing unsolved cases.

"Lori would never harm her children, ever," Shiflet tells 48 Hours correspondent Jonathan Vigliotti.

Cox agrees. "She would never harm her children," she said. "And I know her."

Adds Shiflet: "She's not a monster, she's not a heartless, cold-calculated murderous type of person."

Lori Vallow
Joshua “J.J.” Vallow; Lori Vallow; Tylee Ryan
| Credit: Rexburg Police Department

The comments echo those of Vallow's niece, Melani Boudreaux, who previously said in an interview with Phoenix TV station KPHO that Vallow is protecting the kids "from the people that are trying to harm them." She did not say who or why someone might be a threat.

Lori Vallow's mother and sister are publicly defending the Idaho mom for the first time in the midst of a high-profile missing children case.
Lori Vallow's sister Summer Shiflet, left, and mother Janis Cox
| Credit: CBS News/YouTube

Despite growing concerns raised by law enforcement, Boudreaux said she expects to see the kids again. "I absolutely do," she said. "I don't have the answers as to why she does the things she does, and why she hasn't spoken as to where they are, but I can't judge her because I don't know what she's going through. I can only love and support her, and hope for everyone to be safe."

Vallow, Husband Allegedly 'Abruptly Vacated' Home

Police began searching for the two kids in late November, after the grandparents of the adopted J.J. raised concerns. Vallow and Daybell then "abruptly vacated" their Rexburg home as authorities prepared to serve search warrants, according to police; in late January the couple was located without the children in Kaua'i, Hawaii, where Vallow was served and defied an order to produce them.

Lori Vallow and her husband Chad Daybell in Hawaii
Chad Daybell, at left, and Lori Vallow
| Credit: Eric Grossarth/ EastIdahoNews.com

J.J., was last seen Sept. 23 at his school in Rexburg. Lori subsequently withdrew J.J. from the school, telling the principal she wanted to homeschool him.

The last image of Tylee dates to a Sept. 8 day trip Tylee made to Yellowstone National Park with her mom, brother and uncle, according to the court filing in Madison County, Idaho, that led to Lori’s arrest in the children’s disappearance.

Police say they have documented inconsistent statements from Vallow and Daybell about the children's whereabouts. Daybell, an author of religious-themed doomsday writings, has not been charged in connection with their disappearance.

Asked by Vigliotti, "Do you think they're in a bunker somewhere?," Vallow's mother responds, "I think that's possible."

48 Hours: The Missing Children of Lori Vallow Daybell airs Saturday (10 p.m. ET/PT) on CBS.