Allison Holker's NDA Deters tWitch's Family From Writing Book About Him

tWitch's family is furious with Allison Holker for writing a book about him ... but his widow took steps to make sure she's the only one who could make money off his name.

TMZ obtained the NDA tWitch's family says Allison had them sign in order to attend his funeral, and it essentially financially blocks them from doing what many people assumed she's doing ... namely, collecting any money from a memoir about their relationship and his death.

Under the NDA, tWitch's family and others who signed it agreed to hand over any proceeds from interviews, books, articles or programs about him straight into Allison's pocket ... which is a pretty strong deterrent.

tWitch's family has been ripping Allison online for going on a media blitz to promote her new book ... pushing back against her claims that tWitch was a drug addict, and blasting her as a money-hungry opportunist.

The NDA from the funeral was also a source of family outrage.

Allison's oldest daughter, Weslie Fowler, is defending the NDA ... arguing its purpose was to keep people from photographing his body. Weslie even said when tWitch was alive, people who attended certain family events would have to sign NDAs.

That may well be true, but the document also positions Allison as the only one able to profit off tWitch's life story ... however, she's been clear publicly that 100% of the proceeds of her sales from her book will go to the Move with Kindness Foundation, which was started in her late husband's honor.

TMZ obtained an excerpt from Allison's forthcoming book, "This Far: My Story of Love, Loss, and Embracing the Light," in which she explained the reason behind the NDA.

Allison says Stephen "tWitch" Boss' family wanted a bunch of people she didn't know to attend the open-casket portion of his funeral ... but she whittled the number down to 13 people and made it clear no photographs or videos would be allowed.

She writes ... "I couldn’t risk any photo getting leaked to the press or splashed across social media platforms and becoming the lasting image that people would have of him. My children absolutely, positively could not stumble upon such a photograph at some later date.

"At the advice of my lawyers, I insisted that everybody who viewed the open casket sign a nondisclosure agreement to protect Stephen's privacy, a requirement that exasperated his family. 'If we have to sign NDAs,' they said, 'then everybody has to sign NDAs.' 'No problem,' I replied."