"[They] were stupid and they'd ask, 'Well, why'd you leave her alone'? And I would just start crying at school," she told ABC News. "So it made me think, 'Well, it is my fault, because all the kids think it is.'"
"She said, 'If I hadn't left her, she would still be here," recalled Tracy Biggs. "And that's when I first started telling her: 'You're just a little girl, you couldn't have stopped it from happening.'"
Even though she heard those words from her mother many times, Kimber said it took six years before she truly believed that Mikelle's disappearance wasn't her fault.
"It wasn't until I got...older...more mature, that I really was [like], 'OK. It was not my fault. I couldn't have done anything,'" she said.
Five years after Mikelle's disappearance, her family shifted its focus from searching for Mikelle to hunting for her killer. The case remained unsolved.
To try to get some closure, they held a funeral for their beloved daughter and buried an empty casket, which finally brought Kimber and her family a little peace.
"There wasn't such an empty space..." Kimber told us. "There's a hole in our family now, but I think that hole has kind of healed a little bit."
When Mikelle went missing, Kimber became the oldest child, playing a major role in her younger sister's life. Lynelle, the youngest sibling, is about the age that Mikelle was when she went missing.
"I hang out with Lynelle all the time," Kimber said. "She sleeps over at my apartment once in awhile and we have a girl's night. And I...love being a big sister to her, because I don't get to have that anymore."
Kimber yearns for her older sister and for a relationship that will never be.
"One of my good friends, she has an older sister. And they're about the same age difference as me and Mikelle. And ...sometimes she complains about, 'Oh, I hate having an older sister.' And I was like, "You don't know how lucky you are,'" Kimber said.
If you have information that might help solve the mystery of what happened to Mikelle Biggs, please contact the Arizona Police Department on their website, http://mesaaz.gov/police/, or by phone (480) 644-2211.