Jaafar Jackson, who was 12 when his uncle died in 2009, said he immersed himself in archives of footage to prepare for the role, while also drawing on his own memories.
"One of my favourites is when I first saw him on stage performing in New York for the first time, which was the best experience for me," he told Reuters along the red carpet on Friday night.
Jackie Jackson, who performed with the young Michael Jackson in the Jackson 5 and also pursued his own music career, said the portrayal of his brother was so convincing that during the film, he would at times forget that he was actually watching Jaafar.
"He became Michael in the film," he said.
The film directed by Antoine Fuqua follows the rise of the self-proclaimed King of Pop from lead singer of the Motown group Jackson 5, made up of him and his brothers, to Michael striking out on a solo career with ubiquitous hits like Thriller and Beat It. The film highlights the iconic outfits that came along with them.
The biggest opening for a musical biopic is $US60.2 million ($A85.1 million) in the United States and Canada for 2015 film Straight Outta Compton.
The Box Office Pro website projects that Michael, which opens in theatres on April 24, will top that, with sales possibly exceeding $US80 million ($A113 million).
However, any mentions of the most controversial aspects of Jackson's story - child molestation allegations - were removed after lawyers for his estate realised that a legal settlement with an accuser barred discussion of the accusations in the movie, according to a source with knowledge of the production.
Jackson, who maintained his innocence, was acquitted in 2005 in a criminal trial.
After his death, further civil claims were brought by other accusers, but Jackson was never convicted of child sexual abuse in a criminal court or found liable in a civil case.
Asked about omitting the controversial parts of Jackson's life from the film, producer Graham King, whose credits include Bohemian Rhapsody, said he "spent a lot of years digging into everything, part of Michael's life," and was happy to tell this "celebration story".
