Like 85 million other television viewers around the world, Rosalind Cruz wasn’t expecting to see her sister onstage at the 1973 Oscars when Marlon Brando won Best Actor for “The Godfather.“

“Hello, my name is Sacheen Littlefeather. I’m Apache and I’m president of the National Native American Affirmative Image Committee. I’m representing Marlon Brando this evening and he has asked me to tell you … that he very regretfully cannot accept this very generous award. And the reasons for this being are the treatment of American Indians today by the film industry … “

Rosalind, then 15, gawked in disbelief from their grandparents’ home in rural Salinas, Calif., as Littlefeather — who she knew as her 26-year-old sibling, Marie Louise Cruz — performed what would become the most pivotal 75 seconds of her acting career.

But there was one problem. “She lied,” Rosalind, now 65 and living in Lake County, Montana, told The Post.

Now, weeks after Littlefeather died of breast cancer at age 75, Rosalind reveals to The Post’s JOSHUA RHETT MILLER what really happened — and who “Sacheen Littlefeather” really was.

We were mortified.

Before Marie left for the Academy Awards, she told us: “Just watch it. I can’t tell you anything more, but just watch.” A few years earlier, my older sister had started her acting career at the American Conservatory Theater in San Francisco, but she was still desperately trying to make a name for herself.

So when we saw Marie identify herself as “Apache,” my grandparents and I were blown away. I will never forget how we just stared at each other absolutely awestruck.

The shocking moment marked the first time, to my knowledge, that my eldest sister claimed she was part of a tribe — a bogus identity she deliberately concocted while wearing a borrowed buckskin dress for maximum impact.

My sister Marie, who had already spent a year in a mental hospital after trying to kill herself when she was 19, then assumed that fake image for nearly 50 years. Her elaborate ruse denigrates the memory of our late parents, Manuel Ybarra Cruz and Geroldine Marie Cruz, who were native Californians with no ancestral tribal ties.

I’m now ready to restore their honor by finally telling the truth: Our family has no known Native American heritage, including links to Arizona’s White Mountain Apache and Yaqui tribes, as Marie falsely claimed. Our father’s Hispanic lineage traces back to Mexico, while Mom’s ancestors were French, German and Dutch.

Like myself, my two older sisters, Marie and Trudy, were born and raised in Salinas, where we lived in a two-bedroom home with my parents. This was not the rickety shack that “Sacheen” started describing to reporters in the mid-’70s. The house was directly behind our parents’ custom saddle shop, Cruz Saddlery, in the town’s Santa Rita section, where we kept our horse, Zurc.

As a family living in rural California, we grew our own fruit and vegetables; figs were Marie’s favorite. We also sewed our own clothes, which is where Trudy and I believe Marie got the inspiration for her Native American alter ego. The thread and ribbon we used back then was manufactured by Sasheen Ribbon Company. She just took the second ‘S’ out and put a ‘C’ in there. That’s all she did to start living this fantasy.

I’m still unsure as to what truly inspired Marie to choose “Littlefeather,” a surname she adopted after high school to “reflect her natural heritage,” her official website claims. But many dramatic stories she told over the years were just baloney — like the one about our deaf father giving her that name as she danced around him with a feather in her head, or claiming that he was a violent alcoholic.

Those tales of torment or allegations of abuse came years after Dad died of cancer in 1966 at just 44 years old. They became increasingly vicious, too — like when Marie claimed he tried to run her over with his truck. But those were just more phony narratives from a pathological liar.

Our mother developed dementia around 1978 and couldn’t defend herself. She passed away in 2009. Meanwhile, Marie was busy manipulating everyone with her American Indian act, lying so frequently that Trudy and I couldn’t keep up. Yet Marie’s unchallenged accounts never included a word about her sisters. We simply did not exist.

We lived out in the country and didn’t have many visitors, so no one shamed our family in public. Even so, Marie did her best to embarrass the Cruzes forever when she appeared in Playboy about seven months after the Oscars. We were a conservative family, we went to Catholic schools — we weren’t ready for all of this.

Marie’s sham claim of Native American heritage was nothing more than an opportunistic way for an aspiring actress to bust into Hollywood. She worked her way into Brando’s orbit while living in San Francisco, where she spotted “The Godfather” director Francis Ford Coppola as he moved into an old Victorian house in the Pacific Heights neighborhood. Marie introduced herself and struck up a friendship with him.

With Coppola’s help, she then seized on Brando’s growing interest in Native American advocacy by writing him a letter claiming to be an Indian activist. Always the opportunist, Marie ultimately managed to win over the movie star by professing to be someone else. She realized Brando had already advocated for the Black Panthers and was now switching gears, getting into American Indians. She knew that.

I was embarrassed more than anything else when I saw my sister in that unfamiliar dress on Hollywood’s grandest stage. Now we’re pretending to be Indians? You just don’t do stuff like that. But Marie saw it as a way to advance her career and worm herself into the industry.

I first tried to reveal this nasty secret about 15 years ago, even reaching out to “The Oprah Winfrey Show” at one point. I wanted to expose Marie as a mentally unstable person unfairly trashing our family with the same old deception. But who was going to listen to me? She was the big saint selling the act to people buying the tickets.

I hadn’t talked to Marie in 13 years when my friend texted me to check the news on Oct. 3, a day after she passed away. Trudy and I had dealt with her mental illness our entire lives and I couldn’t take it anymore. Over the years, Marie ramped up her hurtful slander, deeply damaging our family solely to feed her fraudulent persona. She never did mention, however, the inconvenient fact of her two biological sisters not being Native Americans just like her.

Marie also maintained she was “red-listed” because of her Oscars speech — unable to land even the smallest role due to the direct orders of no less than J. Edgar Hoover. But my sister was once again blinded by her grandiose narcissism. Hoover died on May 2, 1972, nearly 11 months before “Sacheen” took that stage.

Hoover gave orders from the grave, apparently. Now do you see how her mind worked? And no one challenged her on it.

When I learned last month about the Academy formally apologizing to Marie for being blacklisted following her Oscars stunt, all I could think was they got swindled. And the funny thing? The Academy Museum honored her as Native even though she’s not. Our family is not Native American. Can you imagine how stupid those guys look?

[The Academy Museum acknowledged claims disputing the late actress’ background on Friday in a statement to The Post. “This is something both Littlefeather and the Native American community have addressed continuously since the 1970s. Native American and Indigenous identity is deeply complex and layered, especially in the United States, and these communities have long battled erasure and misrepresentation. With the support of its Indigenous Alliance — an Academy member affinity group — the Academy recognizes self-identification.”]

Marie kept her scheme going until the very end, telling the captivated Los Angeles crowd about how she took the Dorothy Chandler Pavilion stage in 1973 with dignity, grace and humility as a “proud Indian” woman. Ironically enough, she insisted she had to “speak the truth” all those decades ago.

Marie, who had been diagnosed with Stage 4 breast cancer years earlier, then accepted the Academy’s misguided mea culpa and asked all Native Americans in attendance to stand. She then referenced “our people,” praising them as survivors like herself while insisting that standing for truth would keep her voice alive.

My sister’s final words at that so-called “Evening of Conversation, Healing and Celebration” were “I remain Sacheen Littlefeather.” You know, why give up the part after 50 years?

The purpose of me coming forward now is to bring my parents’ honor back and to tell the truth. By doing that, I’ll naturally unmask my departed sister. White Mountain Apache tribal leaders have no record of any “Sacheen Littlefeather” — the fictional character Marie created to fill a huge void because she didn’t like herself. May she rest in peace, although I hope she grasped the agony she caused my family with her lifetime of lies.