The singer's attorney Blair G Brown has released a statement criticising the executors of Michael's will - John McClain and John Branca.
"The negative media campaign generated by the executors and their agents has been relentless," Brown said on behalf of Janet, Rebbie and Randy Jackson E! reports.
"In recent weeks, the media has received preposterous reports - all now proven to be false - of a purported kidnapping of Katherine Jackson and of physical and verbal abuse of a child.
"The executors and their agents also recently issued a notice barring Janet, Randy and Rebbie from visiting their 82-year-old mother and Michael's children.
"The effect of that notice not only is to damage fundamental family relationships, it is also to isolate Katherine Jackson from questioning the validity of Michael's will.
"It is important to stress that Janet, Randy and Rebbie have questioned the validity of the will with no financial motive whatsoever - they stand to gain nothing financially by a finding that the will is invalid. That point is worth repeating - they stand to gain nothing financially by a finding that the will is invalid.
"Michael's children will be the beneficiaries of Michael's estate. What will be gained by a finding of invalidity is that the executors will be replaced and the estate and the guardianship will be managed in a manner that is in the best interests of the children, which is what Michael wanted.
"The individuals who have the most to lose by a finding that the will is invalid are, of course, the executors and those on the executors' payroll. Janet, Randy and Rebbie will continue to press forward in their search for the truth in order to carry out the wishes of their brother Michael."
Janet had been falsely accused of slapping 14-year-old niece Paris Jackson, while Katherine recently denied claims she'd been kidnapped.
A spokesman for Branca and McClain denied that the will was fraudulent.