"I think the loss of my uncle has hit her at a different level," he said of his cousin Paris, who remains in hospital after trying to cut one of her wrists and taking 20 Motrin (ibuprofen) pills.
"It's tough. She was daddy's girl. My uncle was her world," he added.
The 34-year-old, also a singer/songwriter, was the second Jackson family member to testify after Prince Jackson took the stand Wednesday, recounting for the first time the harrowing day of his father's death.
Michael Jackson's mother Katherine is suing tour promoter AEG Live, accusing them of negligently hiring Conrad Murray, the doctor convicted of involuntary manslaughter for giving the singer the drug that killed him.
Murray was found guilty in 2011 of administering the anesthetic propofol to Jackson to help him with chronic insomnia, as he rehearsed in Los Angeles for the planned "This Is It" comeback tour.
T.J. Jackson, the son of the late King of Pop's brother Tito, was made co-guardian last year of his three cousins Prince, Paris and "Blanket," now 11 years old.
Testifying in the AEG trial's ninth week, he described 83-year-old family matriarch Katherine Jackson as "the queen" and "the CEO," adding: "I help her do the daily stuff."
He said he had become extremely close to his uncle after his own mother died when he was 16.
"My world crumbled and my uncle Michael saved it," he said, at one point taking several seconds to compose himself after he was asked how much the late singer meant to him.
"He was just everything," he said.
Paris Jackson was rushed to hospital after trying to commit suicide at the family home in Calabasas, northwest of Los Angeles, on June 5. She had previously been treated for depression, and remains under medical supervision.
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