• Alex Blum, 71, and his three biological brothers all gather together for the first time in midtown.
• Blum, adopted at birth, found his long-lost siblings through a DNA test.
• Their mother, Lee Hart, told her younger sons their older brother died at birth, a story they doubted.

On a recent sunny morning in midtown, Alex Blum joined his three younger brothers around a plush banquette at a bistro off Grand Central.

The men, who range in age from 63 to 71, playfully ribbed one another, gushing about the grandkids and stealing pancakes off each other’s plate. It looked like they’d all known each other for a lifetime, but it was actually the first time they’d all been together — and The Post was there exclusively to witness it.

Blum, 71, always knew he was adopted, but his three brothers never knew of his existence. About a dozen years ago, he went looking for more information about his biological family and joined 23andMe.

“My life began with a mystery, a question, and since then, the world has always seemed to me like a Chinese box, a problem to solve, a puzzle, a search for somewhere to belong, a life story without history or context,” Blum writes in his new book “An Accident of Birth: A Story of Adoption and Identity” (UnCollected Press, out now).

After about five years on the now-defunct genealogy platform, he heard from a stranger named Brook. They shared 20% of their DNA, meaning she was likely a niece or grandchild. She would turn out to be the daughter of his youngest brother, Pete.

“It was the moment that changed my life,” writes Blum, a happily married father-of-three who lives in San Diego and works as a filmmaker and consultant. “The next thing I knew I was the oldest of four full brothers.”

In 1955, Lee Hart was just 20 years old when she had an affair with a married man named John Stanton and got pregnant. She had little choice but to give the baby up for adoption.

A wealthy, older Upper East Side couple, John and Nancy Blum, adopted the tot and named him Alex. His upbringing was privileged but lonely, despite having an older brother who was also adopted.

John worked in marketing at Macy’s and Unilever. Nancy busied herself with historic preservation organizations and conservation causes. They were cold, distant and WASP-y, always jetting off to boozy black tie events, leaving Blum and his brother with a live-in nanny. .

It was like “living with strangers disguised as parents,”he writes. He always had a “kind of empty feeling,” a yearning for a sense of belonging that never came.

Meanwhile, John Stanton divorced his first wife not long after Blum was born. Within two years, he and Lee were married and expecting another child of their own. Hank was born in 1957, followed by Pete two years later and Bill two years after that.

Stanton was a global executive for a soft drink company, and the family were jets-setters in the kids’ youth, zigzagging the world with stops in Africa and the Alps before settling in Germany. 

Then, in 1967, he caught a severe infection and died suddenly in an Italian hospital at just 52-years-old.

Lee went on to raise the boys as a single mom in hardscrabble Darien, Conn. She sold real estate and sent her sons to public school while battling her own demons. She was erratic and unpredictable and struggled with alcoholism. She told the boys that they had an older brother who’d died in childbirth, but the story never quite rang true.

“Intuitively, something never gelled with that, even as a seven-year-old,” Pete Stanton, 67, writes in the book, which features first person vignettes from all of the brothers. “I kept thinking — even in my youth — that there was something more to the story.” 

Indeed there was. His oldest brother was alive and well, swirling about East Coast society. Blum summered at a country home in Connecticut and attended the prestigious Choate boarding school, his adoptive father’s alma mater. As a senior, John Blum had taunted a freshman named John F. Kennedy there, making the future president run laps around the track in the middle of the night.

When Pete’s daughter initially found Blum in 2019, something clicked for him.

“I immediately knew it was our brother — and that it was the child mom was tortured by giving up — that she had been calling her ‘first son/still born child,’” Pete writes.“It was like the tumblers all fell in place.”

As the coffee and the laughs kept flowing at breakfast, Alex brandished an envelope he protected as if it contained the nuclear codes.

Carefully unfolding the protective layers, he read aloud the two-sided note, handwritten in pencil script on a faded scrap of yellow lined paper. One of his nieces had found it in an old lockbox of relics years ago and given it to Alex, believing that his mother had written it for the son she’d given away.

“There is only one sin beyond forgiveness and that is fear of a code. To fail someone who needs you, to starve someone of love because you fear to offend against a code, that’s a mortal sin,” read one side. “You never come to terms with yourself again — never. And that is the origin story of my life,” read the other.  

Blum treasures the note, the only thing he has from his biological mother. “[It’s] essential to my understanding of the story, my story, and my brothers’ story as well,” he said.

Brother Hank, 69, was at first incredulous at the thought of another one of them out there — until he first saw Blum’s photo.

“My wife took one look and said, ‘That’s your brother.’ It was right away,” recalled Hank, who works in publishing in Baltimore and produced the book.

Pete recalled that when he first talked to Alex on the phone years ago, he immediately felt like family.

“it was like we had known each other forever — even though we just met,” said the Florida-based father of two and entrepreneur in the financial industry. “It was like an old friend.”

“The amazing thing to me was how easy it was,” added baby brother Bill, 63, a father-of-four girls who retired from commercial lending and splits his time between New Jersey and Europe. “[We] just created a new normal – with someone we haven’t seen in a long time.”

Now, it’s not just about making up for lost time, but maximizing the present. They have regular Zoom calls, where they laugh and debate the quality of the “Godfather,” “French Connection,” “Indiana Jones” sequels. They hope to get together in person again in the next year.

“There’s a little bit of Alex in each of them,” Blum’s wife, Andrea, told The Post.

At their bistro meeting, the oldest brother marveled at his good fortune.

He said, “I got three best friends overnight.”