Michael Cavallari appeared in a scene of Laguna Beach with his sister

By Michele Corriston
Updated December 08, 2015 03:30 PM
Source Instagram

Kristin Cavallari and her brother Michael, who has been missing for almost two weeks, spent part of their childhood apart – though a source has told PEOPLE the fashion designer “was always very protective of him.”

As recounted in MTV’s 2005 companion book Laguna Beach: Life Inside the Bubble, the siblings were separated by their parents’ divorce when Kristin was 8 years old. She stayed in Colorado with her mother, and Michael eventually moved to California with their father.

But when she was a young teenager, her mother remarried, relocating the family to Chicago – a move that Kristin says motivated her to act out.

“Things hadn’t been so bad back when she and her brother Mike were still living with their mom in Colorado. Dad had come back almost every weekend to be with them,” according to the book. “But now that he and Mike were living in Laguna Beach and Kristin had been forced to move to Chicago with her mom and her mom’s new husband, things were going from bad to worse.”

After getting caught sneaking out of the house during the summer before ninth grade, Kristin was sent to live with her father and brother in Laguna Beach, California – where she got her first taste of fame as the star of MTV’s eponymous reality series during her junior year.

Michael, now 30, even appeared in a season 1 scene in which Kristin, 28, ran the idea of asking for a new car by him.

“Keep dreaming,” he told his sister, laughing.

Michael’s 2014 Honda Civic was found abandoned off a Utah highway on Nov. 27, and authorities are treating the case as an open missing person investigation. On Tuesday, a representative for the Grand Prairie County Sheriff’s Office told PEOPLE that police are still actively searching for him.

“I appreciate all the concern, thoughts, and prayers that have been extended to me and my family,” Kristin wrote Monday on Instagram. “We remain hopeful and I will share any news that we receive. I have such gratitude for all those that have shared their support.”

Michael was arrested Nov. 23, reportedly with a loaded shotgun, and briefly held in an Orange County jail. He lives in San Clemente, California, but his mother is still in Chicago – along with Kristin, who calls the Windy City home again and is married to Bears quarterback Jay Cutler, with whom she has three children.

“Her brother was always a troubled kid and there was always drama with him,” a source told PEOPLE of Michael on Monday. “She was always trying to help him. He had been trying to produce reality shows awhile ago, but it didn’t work out.”

Anyone with information regarding Michael should contact the Grand County Sheriff’s Office at 435-359-8115.