(NewsNation) — It’s been more than 3 1/2 years since Tylee Ryan and Joshua “JJ” Valow first went missing, and on Monday, their mother will stand trial, charged with their murders.

Tylee Ryan was 16 when she disappeared and JJ Vallow was 7. Prosecutors say their mother, Lori Vallow, and her now-husband Chad Daybell used doomsday-focused religious beliefs to further a plan to kill the children.

JJ and Tylee were found dead on Chad Daybell’s property in June 2020 after a monthslong search for the siblings. Last seen in September 2019, they were reported missing by extended family members in November of that year, according to ABC News.

Lori Vallow and Daybell are also charged in connection with the death of Chad Daybell’s previous wife, Tammy. Jury selection in Lori Vallow’s case is set to begin Monday.

Tylee Ryan

Tylee Ryan, born on Sept. 24, 2002, was described by family members as “loving, and constantly laughing,” according to Idaho news outlet KPVI.

She loved spending time with friends and watching “The Office,” the news outlet reported, but “more than anything” she loved hanging out with little brother JJ and older brother Colby Ryan.

“She’s so witty … even for adults, like, she would have me laughing,” Echo Itaaehau, a family friend, told local Phoeniz news outlet KTVK. “She could be sassy, but just super sweet and tender-hearted.”

Lori Vallow and her third husband, Joseph Ryan, divorced when their daughter Kylee was a toddler, KPVI reported. In 2018, Joseph Ryan died of an apparent heart attack.

Marriage records obtained by the East Idaho News show Lori Ryan married her fourth husband, Charles Vallow, in 2006, and he became Tylee’s stepfather. Charles Vallow was ultimately shot and killed by Lori Vallow’s brother, Alex Cox, in 2019.

Cox, who died of natural causes later that year, claimed self-defense.

Many of Tylee’s teen years were spent in Arizona, Fox 10 Phoenix writes, where she went to Perry High School for a short time.

Tylee was last seen on Sept. 8, 2019, on a trip to Yellowstone National Park with J.J., her mother and Cox. She had moved with her family to Rexburg, Idaho, a week before going missing.

A former friend of Lori Vallow, Melanie Gibb, told police Vallow said Tylee took classes at Brigham Young University-Idaho in Rexburg, according to The Arizona Republic. However, BYU-Idaho told police the girl was never enrolled.

Police noticed Tylee’s disappearance on Nov. 27, 2019, KSL-TV said, during a welfare check on JJ where authorities executed a search warrant at Lori Vallow’s Rexburg, Idaho, home.

Joshua “JJ” Vallow

Charles and Lori Vallow adopted JJ in 2014. According to the East Idaho News, JJ is the biological grandson of Charles Vallow’s sister Kay Woodcock.

He was last seen on Sept. 23, 2019, at Kennedy Elementary School in Rexburg, according to KPVI. The day after, Lori Vallow informed the school JJ would not be attending anymore. Instead, she said, JJ would be homeschooled.

Days before he went missing, the East Idaho News said, Ring doorbell camera footage shows the boy running outside and playing with a friend.

Woodcock in November requested a welfare check on the boy, Law and Crime said. Police were not able to locate him.

Before moving to Idaho, JJ went to school at Lauren’s Institute for Education in Gilbert, Arizona. Teachers there who met JJ told media that he was a “loving little boy.”

“He just brought so much joy for us here,” Lori Kleinman, director of academy administration at Lauren’s Institute for Education, said in The Arizona Republic.

JJ, who had autism, had a tough time engaging at first, but ended up making a lot of progress, Nicole Rapp, his teacher for his first two years at the school, told the newspaper.

“I remember him just being so excited to come here to school, running down the hall,” Rapp said. “He could not wait for class, to meet teachers, play along with friends. JJ loved it here, he loved being around his peers. The classroom was his home.”

Tylee’s best friend, Vaisia Itaaehau, said when she thinks of Tylee and JJ, she thinks of “fun and love.”

“They both had unconditional love for each other. … I’ll always remember them like that,” Itaaehau told CBS, adding that JJ was “the sweetest soul” she had ever met.

Tammy Daybell

Tammy and Chad Daybell were married on March 9, 1990, in Utah, her obituary states. Years later, in 2004, the couple founded Spring Creek Book Company.

“Tammy wore many hats as their company grew, juggling her role as Chief Financial Officer with designing book covers,” her obituary said. Later, in 2015, when she and her family moved to Salem, Idaho, Tammy Daybell became the assistant librarian at a local middle school, then at an elementary school.

“She was treasured and loved by the staff and students alike,” the obituary said. Active in the Church of Jesus Christ of Latter-day Saints, Tammy Daybell ended up becoming Stake Girls camp director and Young Women president.

But Tammy Daybell’s true focus was her husband Chad and their five children, the obituary said. At the time of her death, she had two grandchildren and a third on the way.

The Daybell children told CBS’ “48 Hours” they declined an autopsy for their mother after a coroner told them her death appeared to be from natural causes. CBS wrote in 2022 that the five remained “steadfast” in their belief that their father didn’t have anything to do with the death of their mother.

Investigators, on the other hand, grew suspicious after Chad Daybell married Lori Vallow on Nov. 5, 2019 — just two weeks after Tammy Daybell died. Her body was later exhumed as part of the investigation, but the results of the autopsy have not yet been released to the public.

Friends of Tammy Daybell later told a local news outlet something wasn’t right in the days leading up to her death. They told KSL-TV their friend seemed “distant,” which was different from how she usually acted.

“She smiled nonstop, she was out to do her very best and then some,” said Mandy Fowler, Tammy Daybell’s co-worker. “She was the most amazing person ever. … You only meet someone like her once in a lifetime.”

The Associated Press contributed to this report.