EXCLUSIVE: Kristin Cavallari Opens Up About Her Brother's Death: 'I Couldn't Collapse, I Had a Newborn Baby'

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Kristin Cavallari says she has "good days and bad days" dealing with her brother's tragic death.

ET sat down with the 29-year-old former reality star to discuss her new book, Balancing in Heels: My Journey to Health, Happiness, and Making it all Work, and asked how she is coping after her brother, Michael, was reported missing in December and found dead days later.

"I've really found a lot of strength in my kids," the mother of three tells ET's Jennifer Peros. "I had Saylor at the same time my brother went missing, and then ultimately passed away, and so I couldn't collapse. I had a newborn baby to look over."

WATCH: Kristin Cavallari on Her Car Crash Injuries: 'Not Being Able to Hold My Baby Is Absolutely Killing Me'

"I had to be strong for her and my whole family, so, that's what I've been doing," Kristin recalls, adding that her family is doing "OK" now. "I feel for my parents. They've been going through it, but everyone's getting better."

In February, a medical examiner ruled the 30-year-old's death an accident and determined that Michael, who had crashed his car while passing through a mountainous part of Utah, died of hypothermia due to below-freezing temperatures.

As for rumors that Michael, who was arrested for making criminal threats days before he went missing, may have been suffering from bipolar disorder, Kristin says those claims are false.

"He was never diagnosed, so that's actually not true," she confirms.

EXCLUSIVE: Kristin Cavallari's Brother Michael Arrested Days Before He Went Missing

Now, Kristin wants to remember her brother for the "really funny, great guy" she says he was.

"He had such a good heart. He was the only one who could push my buttons the way he did," she adds with a laugh. "It was like that brother-sister relationship, but he would make me laugh, then push my buttons, and then make me laugh."

In fact, when asked if she sees any of her late brother in any of her children -- 3-year-old Camden, 1-year-old Jaxon, or 3-month-old Saylor -- Kristin says that she does.

"I see a lot of him in Camden, actually," she reveals. "Camden is a very sweet, sensitive soul and that's how my brother was. So, that's kind of nice."

Following her brother's death and her subsequent car accident, Kristin told ET that she had "such an appreciation for life." Find out what she had to say in the video below.