20 Years After Her Big Sister Vanished, Arizona Woman Is Keeping Hope Alive in the Case

At ages 9 and 11, sisters Kimber and Mikelle Biggs were the best of friends — but could not have been more different.

“She was the perfect child, the straight-A student,” Kimber tells PEOPLE of her sibling. “She was very clean and organized, a borderline neat freak.”

“I was the exact opposite,” she says. “I did not like to clean my room. I didn’t care if it was a mess. Us sharing a room was a disaster for her. She put blue masking tape right across the middle of the room and separated it and said, ‘This is my side, this is your side,’ and I was not allowed to get any of my toys — or as she called it, any of my ‘crap’ — on her side.”

But their antagonistic affection for each other was shattered on Jan. 2, 1999, when Mikelle vanished not far from her home in Mesa, Arizona, Kimber says. The two were outside on their suburban street as dusk approached when Kimber turned her back on Mikelle to go inside — just for a second — only to return to find her gone.

The bike Mikelle was riding had been dumped in the street, a tire still spinning, and nearby on the ground lay the quarters she’d clutched in anticipation of the ice cream truck she heard coming their way.

The enduring mystery of her disappearance is detailed in the latest episode of People Magazine Investigates, airing on Investigation Discovery. (An exclusive clip is above.)

“I convinced myself it was my fault, and if I hadn’t left her there it wouldn’t have happened,” says Kimber, now 29, and the mother of a 6-year-old son. “I actually remember crying myself to sleep and thinking that.”

Days later, when Kimber confessed her feelings of guilt to their mom, Tracy, “she told me ‘if you had stayed out there, it could have been both of you, or it could have been you instead of her.’ She made it clear that it was the person’s fault who took her, and no one else’s.”

RELATED: The ‘Eerie’ Scene After Girl Vanishes Waiting for Ice Cream Truck — Her Bike Left Behind

Mikelle’s parents, Tracy and Darien Biggs, found a way forward through their grief by holding a funeral on the fifth anniversary their daughter’s abduction and symbolically burying an empty casket.

“It was, we felt, needed as a way of having closure, or a way of being able to say goodbye to some sort of degree in order to move on,” Tracy, 53, tells PEOPLE. “I was grateful that we did it.”

Kimber found her own small comfort several years later after observing her then months-old son, Trayven, encounter a photograph of her sister for the first time at her mother’s home.

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“He just started laughing and smiling at it for no reason,” she says, recalling that the moment gave her “goosebumps.” “It made me think her knew her, like she was his guardian angel.”

“For years I’ve told myself that Mikelle was watching over me,” she says, “and that really made me feel like she was.”

Becoming a parent also gave Kimber perspective on losing a child, she says. She created a Facebook page, Justice for Mikelle Biggs, initially as “a coping mechanism for anyone who was touched by her life,” but also now serving as a place to collect tips and advocate for child safety and awareness.

The latter received a boost when, earlier this year, a dollar bill found in Wisconsin that purported to carry a handwritten message from her sister — “My name is Mikel [sic] Biggs kidnapped From Mesa AZ I’m Alive” — pushed Mikelle’s story back into the spotlight before authorities quickly judged the note to be a hoax.

RELATED: Could a Former Neighbor & Convicted Sex Offender Be Connected to Disappearance of Mikelle Biggs?

“It is a horrible thing,” says Kimber, “and I don’t appreciate that someone felt the need to do that. But instead of being angry and hating them, I said, ‘Hey, thanks, you gave my sister more attention, you brought her back into the forefront.’ “

Two Mesa police investigators who worked on Mikelle’s disappearance, Sgt. Kevin Baggs and retired detective Jerry Gissel, tell PEOPLE that an incarcerated sex offender who lived in the neighborhood, 61-year-old Dee Blalock, has denied involvement but remains a person of interest.

Blalock has never been charged in the case.

“I do feel it will be solved,” says Kimber. “There’s no way of knowing when, but I personally feel like I will get the answers. I believe it is one of the purposes in my life.”

“I would be overwhelmed with gratitude if I just got answers, if I just knew what happened,” she says. “But I won’t give up on getting justice either.”

People Magazine Investigates: Gone in 90 Seconds is available to watch online now on Investigation Discovery.