In a devastating act of betrayal, a new tell-all is claiming to expose the secrets of media mogul Oprah Winfrey’s mysterious relationship with longtime fiancé Stedman Graham.

And the turncoat author is Stedman’s own niece, The National ENQUIRER can 

Courtney Shelton – the 36-year-old daughter of Stedman’s sister Anita – is shopping the book to literary agents in a bid to get it on to bookshelves, a publishing source exclusively told The ENQUIRER.

“I was very shocked at the picture she painted of her family,” the source said. “She describes – in a written pitch – Stedman’s ‘abuse’ toward his family and how Oprah turned a blind eye to 

“She also claims some members of her own family treated Stedman’s daughter from a past marriage very badly.”

Stedman, 63, a motivational speaker, became engaged to marry Oprah in 1992, but the pair never wed and now say theirs is more a “spiritual union.” His daughter, Wendy, was born in 1975, before his divorce to ex-wife Glenda.

The ENQUIRER tracked down Atlanta-based Courtney to ask her about the exposé, and she confirmed she’s peddled it to “a select number of agents” over the last three months, but has yet to ink a publishing deal.

“This is what I do,” she told The ENQUIRER. “I work as a chef, but I’m a writer by trade.”

While Courtney did not elaborate on the book’s juicy contents, she said of 60-year-old Oprah: “At first, I just thought she’s my uncle’s girlfriend – but she wasn’t good for the family. The media frenzy made our problems much worse than what they really were, and she did nothing to help us.”

A Winfrey insider told The ENQUIRER: “It’s a betrayal because Oprah and Stedman did nothing to bring this on, but they obviously can’t control the actions of his family.

“Oprah had always been close to Stedman’s family members and never turned ‘a blind eye.’ When Wendy had some drinking problems in 2008, Oprah helped place her in a rehab facility.

“And before his mom passed away in 2010, Oprah and Stedman would often visit her at her home in New Jersey.

When Stedman was honored a few years back in his hometown, Oprah came out and encouraged the rest of his family to be there. The fact is he never abused anybody. Stedman’s one of the most honorable guys around.”

The book may detail the shocking stories about the duo previously reported by The ENQUIRER, including Oprah’s alleged drug use, jealousy, cheating, vicious gay rumors and explosive comments made by her dad!

According to an insider, Stedman “was furious” with Oprah’s ex-lover Randolph Cook, who revealed she used cocaine in the 1980s. The alleged drug use happened only a year before her talk show burst onto the national scene and months before she met Stedman, who deplores alcohol and drugs.

.What’s more, sources said Oprah and Stedman underwent secret counseling sessions.  One session “exploded in a torrent of confessions involving jealousy and cheating,” and at one point Oprah told him, “I don’t love you anymore,” sources said.