Oprah Winfrey has made billions sharing her thoughts and opinions with the world – her favorite things, her favorite books, her adoration of her half-dozen dogs and her relationship with her best friend, Gayle, to name a few.

But one subject has always been off limits: Winfrey’s own family.

Mother’s Day came and went with Winfrey devoting an hourlong show to moms – Maria Shriver’s, Demi Moore’s and Vanessa Williams’ among others – without one mention of her own.

Now, as Father’s Day approaches, Winfrey has discovered that her dad has reportedly been writing a tell-all about her.

Winfrey has been betrayed before by family members willing to shred her privacy for a quick buck – including a now-dead half-sister who sold the story of Winfrey’s teen pregnancy to a tabloid for $19,000.

Winfrey’s rise to fame is an astonishing rags-to-riches story.

“She has fascinated me for many years,” Kitty Kelley said when she announced in December that she would write an unauthorized biography on the high priestess of daytime talk, the nation’s first African-American billionairess.

Kelley has vowed to deliver news on the woman most Americans view as an open book. But those details that have already been made public are shocking enough.

Winfrey was born in 1954 to Vernita Lee and Vernon Winfrey, who had a brief fling but never married. She was initially raised on the Mississippi pig farm of her maternal grandmother, Hattie Mae Lee.

Amenities like running water and electricity were scarce, but there was plenty of space, something the driven 6-year-old girl desperately missed when she was sent to Milwaukee to join her mom and half-siblings, Patricia and Jeffrey, in their cramped apartment.

In that neglectful environment, as her mom juggled odd jobs and went on and off welfare, Oprah had no defense against a sly sexual predator – a 19-year-old cousin who raped her. She would also fall prey to a deviant uncle and a twisted family friend numerous times over the next few years.

The trauma turned her into a rebellious teen who lashed out at her mother and everyone around her except her teachers. Somehow, Winfrey excelled in school, partly through the love of reading she learned early in life. She skipped two grades and secured a scholarship to prestigious Nicolet HS in the wealthy suburb of Glendale.

But her spirit was crumbling under the strain, and she often chose to roam the streets rather than go home. By her own admission, she was a “promiscuous” and “wild” young girl.

At 14, she became pregnant and was packed off to her father’s house in Tennessee. Had she stayed with her mom, she would have languished in a juvenile hall for wayward teens.

“I was so ashamed, I hid the pregnancy until my swollen ankles and belly gave me away. The baby died in the hospital weeks later,” Oprah wrote in February for an article on “firsts” in her magazine, O.

“I went back to school and told no one. My fear was that if I were found out, I would be expelled,” she says. “Even when I found the courage to publicly reveal the [sexual] abuse, I still carried the shame and kept the pregnancy a secret.”

When she learned her sister had blabbed about the incident, Oprah told a reporter at the time, “I was shooting [the film] ‘Brewster Place,’ and I got through the shoot, and I went home and got in the bed and cried.”

Last November, while interviewing a teen who had become pregnant, Oprah told her: “I was raped at 9 and sexually abused from the time I was 10 to 14. At 14 years old, I became pregnant.

“I was taken to the hospital, and the doctors said, ‘Are you pregnant?’ I didn’t even know what pregnancy was. I’d been abused all this time, but I really wasn’t even sure that the outcome was you have a baby. So I confessed that I had been having sex with people who had been abusing me for years. The stress of that confession caused me to go into labor, and the baby died.”

Oprah credits her strict father, a former military man, for her strong work ethic and sense of discipline. After her 2-week-old baby died, Oprah disassociated herself from the past and never looked back.

Oprah said her father told her, “What you have done is the past, and you alone get to determine what your future will be.”

Winfrey went on to win the 1971 Miss Fire Prevention and Miss Black Tennessee titles before majoring in media and hitting the radio and TV talk-show scene.

Until worldwide fame made her a target, only two people – best friend Gayle King and constant romantic companion Stedman Graham – were privy to her inner demons.

Winfrey has also spoken of a bout with drugs in her 20s. But her “wild child” teen years are still mostly a mystery – and the book proposal that dad Vernon circulated among agents without Oprah’s knowledge strongly suggests there is more to reveal.

Her mother “said she stayed out all times of night, said she made herself known to boys,” he writes in the outline for the tentatively titled “Things Unspoken.”

“But it was worse than I realized,” he continues. “She had secrets. Dark secrets. Some I didn’t discover till she was a grown woman, till it was too late.”

Vernon hints at a confrontation between him and his then-14-year-old daughter at the kitchen table in his proposal, and owns up to the fact that his fractured relationship with her mom, Vernita Lee, made for a derelict childhood.

“The damage was our fault, her mother’s and mine,” he confesses. “For years we had shuttled our daughter between my home in Nashville and her mother’s home in Milwaukee. That was a mistake.”

Oprah reacted bitterly when news of her 74-year-old dad’s betrayal surfaced, saying she was shocked and stunned to learn he had been penning a memoir.

Vernon, reached last week at the Nashville barbershop he still owns and runs, wouldn’t confirm reports that he had backed off his book after being confronted by an angry Oprah.

“Well, lately I’ve just thought it best to say, ‘No comment,’ ” he drawled politely.

Whatever their problems in the past, Oprah has done right by her parents. They were among the first to benefit when her career began its meteoric rise in 1986, starting with a Best Supporting Actress nomination for her role as Sofia in “The Color Purple,” and the national launch of “The Oprah Winfrey Show,” a retooled version of her local talk show, “A.M. Chicago.”

“The Oprah Winfrey Show’s” profits skyrocketed to $125 million in just 12 months, earning her the unprecedented salary of $30 million, a massive jump from the $200,000-per-year contract she had signed just two years before.

With plenty of cash in the bank, Vernita retired from her job as a hospital dietitian and moved to a luxurious lakefront condo in Milwaukee with $5,000-a-month for life from Winfrey.

Vernon, who asked only for a new set of truck tires, a new TV for his barbershop and a ticket to a Mike Tyson fight in Las Vegas, got everything he wanted, plus 10 scholarships endowed in his name at Oprah’s alma mater, Tennessee State University.

But success also resurrected sibling rifts from the family’s painful past.

In 1990, Oprah suffered what she would later call “her first betrayal,” when half-sister Patricia Lee-Lloyd approached a national tabloid and, according to Oprah, “sat in a room, told them the story of my hidden shame – and left their offices $19,000 richer.”

The one detail about the pregnancy that has never been publicly revealed is the identity of the baby’s father.

It’s not clear whether anyone knows the answer. By all accounts, the baby came at an exceptionally troubled point in her life.

Aside from half-sister Patricia, who died of causes believed to be drug related in 2003, Oprah also struggled to forge a relationship with her half-brother, Jeffrey, who died of AIDS in 1989. Jeffrey never sold any secrets to the tabloids, but he openly accused her of not supporting him financially because she didn’t approve of his gay lifestyle.

A man named Randolph Cook also claimed to be a part of Oprah’s painful past, saying the two shacked up together in 1985 and enjoyed a torrid affair that included drug use, mostly cocaine. He unsuccessfully sued Oprah for $20 million in 1997, alleging that she was blocking publication of his confessional tome. Oprah never went on record about what relationship, if any, she had with Cook, but copped to using cocaine briefly in her 20s during a 1995 taping of her show.

More recently, she has had to deal with Kiefer Bonvillian, 36, who allegedly made a secret recording of an Oprah employee talking trash about her boss, and then threatened to use it in a book unless he was paid $1.5 million. The federal government filed extortion charges against him, but they didn’t hold up in an Illinois court.

gotis@nypost.com