Jermaine and Randy Jackson said the family should be concentrating on the upcoming manslaughter trial of their brother's doctor, which will be going on in Los Angeles when the planned show is staged in Britain in October.
The Michael Forever show is set for October 8 in Cardiff, Wales, and will be broadcast to 30 countries around the world, said his mother Katherine Jackson, sister La Toya and brothers Tito, Marlon and Jackie.
"The world's greatest performing artists" will play at the show, according to its website, although promoter Chris Hunt declined to name names, saying they will announce artists at another press conference in London on August 3.
Jackson's mother said "as many members of my family that are available" will be at the concert, adding: "Michael gave his entire life to the world through his love, his music and his devotion to healing the planet.
"I am positive that this event will be an event great enough in scale, talent and imagination to form a worthy celebration of Michael's life," she said at a press conference in Beverly Hills.
But Randy and Jermaine Jackson -- who tried but failed to organize a tribute show after Jackson's 2009 death -- blasted the timing of the new show, which will coincide with the September manslaughter trial of Jackson's doctor.
"While we wholeheartedly support the spirit of a tribute that honors our brother, we find it impossible to support an event that is due to take place during the criminal trial surrounding Michael's death," they said.
Doctor Conrad Murray, charged with manslaughter for giving the singer an overdose of powerful sedative propofol, is due to go on trial with opening statements on September 20, less than three weeks before the Cardiff concert.
"In light of this, we feel it is inappropriate to be involved with such an ill-timed event and its promoter, Global Live," said Jermaine and Randy Jackson in a statement.
"Furthermore, the decision to proceed with this concert disrespects opinions and wishes expressed in the strongest terms to Global Live during conversations in April when this event was presented to the majority of the family."
Global Live chief Chris Hunt defended the Cardiff plans, saying they would be "worthy of Michael's musical genius ... something fun and authentic that you would envision Michael attending with excitement and joy."
Tickets for the concert in the 75,000-seat Millennium Stadium in Cardiff -- with a fully retracting roof the biggest indoor venue in Europe, according to the organizers -- will go on sale online on August 4, they added.
The concert will raise money for charities including Prince Charles' Prince's Trust in Britain, and the AIDS Project Los Angeles (APLA). (AFP)
