In the letter, the five siblings vow to continue their fight against the executors.
"We are going to take every appropriate action to seek justice and to see to it that the truth be known," the letter reads. "We will hand this over to the proper authorities."
The letter was not signed by Jackie, Marlon and La Toya Jackson, nor did parents Joe and Katherine Jackson sign it.
ABC News requested comments from several Jackson siblings but none of their reps responded immediately.
Here are the top five accusations made in the letter:
The Will Is a Fake:
"The shock of Michael's sudden unexpected passing was overwhelming to our family, to say the least. At that time we couldn't possibly fathom what is so obvious to us now: that the Will, without question, it's Fake, Flawed and Fraudulent.
"Amongst many other inconsistencies in the Will, there is also a conflict around Michael's location on the day he supposedly signed the Will, July 7, 2002. According to what is witnessed in the document, it is impossible and illogical that he could have been in two places at one time. We have evidence that undoubtedly supports and proves that Michael was absolutely not in Los Angeles, California, on the date of his signature reflected in the Will at-hand."
Michael Jackson Despised His Executors:
"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. We know that and you knew that. We believe you relied on the presumption that no one would be so bold as to suggest that you would perpetrate such unconscionable deceit; but you were wrong."
The Executors Are Pressuring Katherine Jackson:
"She is an eighty-two year old woman. Your actions are affecting her health, and on top of that, we've just found out she recently had a mini-stroke. Please understand, she's not equipped to handle the stress load you are putting on her. She feels, as she has said, 'I'm stuck in the middle.' She too knows and acknowledges the Will was forged. She wants to do the right thing, and move in the direction of justice for her son and family, yet she fears the POWERS THAT BE."
The Executors Are Conspiring with Katherine Jackson's Advisers:
"For some strange reason her advisors Lowell Henry, Perry Sanders and Trent [Jackson], although in agreement with her regarding the validity of the Will, are telling her to disregard what she knows as fact. lnstead, her so called advisors are convincing her to let them negotiate 'deals' with Branca and McClain on her behalf, or is it on the behalf of all of you. Her advisors' loyalty seems to be skewed by the percentage you offer them, preventing them from advising her properly."
Sanders responded to CNN that "my only loyalty in this matter is to Mrs. Jackson, and the public record of my success for her speaks for itself."
The Executors Are Part of a Conspiracy Surrounding Jackson's Death:
"We know there is most certainly a conspiracy surrounding our brother's death and now coarse manipulation and fear are being used to cover it up. Your heartless pursuit of wealth, fame and power is at the expense of our family, whose deepest desire is to give to the world a gift of hope, love and unity through our music. Though we have lost our brother, we live and will continue to fight in unity."