Janet, Rebbie, Tito, Randy and Jermaine Jackson have called for the resignation of the late star's estate executors, reports Celebuzz.
The Jackson family claim that administrators John Branca and John McClain faked documents before his death in 2009 in order to gain access to his fortune.
The signed letter to the pair states: "At the time we couldn't possibly fathom what is so obvious to us now: that the will, without question, it's fake, flawed and fraudulent.
"Since the passing of Michael, our beloved brother, you have failed to perform your duties as executors of his estate, but what you have not failed at is taking advantage of a grieving mother, father and a grieving family.
"Our brother told us, in no uncertain terms and without hesitation in the months prior to his death, that he despised both of you and that he did not want either of you to have anything to do with his life or estate for that matter."
The letter accuses them of increasing their own income, and therefore taking away much of the inheritance intended for Jackson's three children and elderly mother Katherine.
Randy Jackson later took to Twitter to confirm the note given to the administrators, writing: "Yes, the letter is authentic."
He added: "We ask that everyone respects that this is a serious matter that will be handled by the proper authorities."
Paris Jackson paid tribute to her late father on the third anniversary of his death last month.