"My heart is shattered & to write this feels so wrong and I don’t even know what to say. Yesterday i lost my brother, best friend & the other half of my heart. There wasn’t a person i loved more on this earth," the model wrote next to a photo of her and Daniel as children.
"There's no words that can do him justice that I could write. To know him was to love him," she continued. "He was the happiest brightest smiley most sunshine human to exist and I’m so happy God chose me to be his sister for his whole amazing life."
Daniel appeared in the 2019 film The Killer Clown Meets the Candy Man as well as the series Mani. He was also the founder of clothing brand Kids Back Home.
Mickelson's death is the latest in a string of high-profile accidental drug overdoses in which fentanyl has played a role. It was determined that Michael K. Williams' death earlier this month was a result of ingesting fentanyl-laced heroin and cocaine. Fentanyl also played a factor in the deaths of Prince, Michael Jackson and Tom Petty, among several others.