Best Friends Who Discovered They're Sisters Now Learn They Have a Brother: 'This Has Been So Crazy' (Exclusive)

“As soon as I saw him, I was like, 'Oh my God, that's me with a beard,’ ” Julia Tinetti tells PEOPLE

About a decade ago, Cassandra Madison posted on social media because she really thought she and two of her friends might be related.

DNA testing confirmed Madison, who had long been looking for more of her biological family, was right.

First, in 2021, she learned that Julia Tinetti, whom she had met in 2013 while working at a Connecticut restaurant, was her biological sister.

And then, this summer, another DNA test confirmed they have a biological brother, too: Madison's childhood friend Thomas Buonaguirio.

“As soon as I saw him [Buonaguirio], I was like, 'Oh my God, that's me with a beard,’ ” says Tinetti, 34, who now works with individuals who have brain injuries and spinal cord injuries.

“This has been so crazy," Tinetti tells PEOPLE. "This whole story has been wild. I didn't think it could get any crazier.”

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Madison and Buonaguirio met when they were in elementary school; they grew up about 15 minutes apart in Connecticut.

Both of them were adopted from the Dominican Republic and someone from the adoption agency that worked with Madison's family had said that she and Buonaguirio could be related. But for decades, Madison wasn't sure.

“Nobody had proof that we were brother and sister,” she says. “Nobody had paperwork.”

Eleven years ago, Madison met Tinetti, who was also adopted from the Dominican Republic. Initially bonded at work, they joked they were sisters and called each other "twins" — but their adoption paperwork didn’t match.

Several years later, a DNA kit led Madison to meet her biological father, Adriano Luna, in the Dominican Republic in March 2019.

At first, Madison was thinking of Buonaguirio and not Tinetti when she told her dad, "Papi, I also found your son!” However, Luna very firmly told her that he had never placed a baby boy up for adoption and she quickly changed the subject.

“That was awkward,” says Madison, 36, an emergency room technician in Virginia Beach, Va.

In 2021, though, Madison's biological father revealed that he and his wife, Juliana, had placed another girl up for adoption. So Madison drove back to Connecticut and insisted Tinetti take a DNA test.

And just like that, the best friends learned they were related — and their family suddenly expanded to include Luna and seven siblings in the Dominican Republic. "Nothing can compare to the happiness I feel," Luna previously told PEOPLE.

The question remained of Buonaguirio, her childhood friend. Madison never mentioned her possible brother in previous interviews with PEOPLE and other news outlets because she couldn't confirm it.

"I didn't want to drag him into something that may not be true," she says. "I didn't want this big mystery. I didn't want to bring it up."

In the spring, out of the blue, Madison’s husband, Aaron, asked if she'd spoken to Buonaguirio. She had mostly lost contact with him but reached back out via social media and asked if he would be willing to take a DNA test.

"I was like, 'Let's just put this thing to rest,' " Madison remembers.

He agreed.

“Of course I was intrigued,” Buonaguirio, a 31-year-old mechanic in Hutto, Texas, tells PEOPLE. “It was kind of a roller coaster of emotion.”

The test showed they were full siblings.

How exactly it was that Buonaguirio was adopted from the Dominican Republic remains unclear. Luna's wife, the siblings' biological mother, died in 2015, and Luna has said he didn’t know the boy existed. So they have pieced together what they can.

They say they have been told their birth parents separated for a time, and they believe their mom moved out of town, delivered Buonaguirio and placed him for adoption under a different last name.

“We still don’t have all the answers,” Tinetti says.

Buonaguirio has a newborn baby boy of his own — and his two American sisters are hoping to visit in November and meet their nephew. Madison is also trying to plan a family reunion in the Dominican Republic, so Buonaguirio can meet his biological father and the seven siblings who live there.

Someday, he says, he will probably take his son.

“It's great to know that I do have family out there,” he says.

For now, he’s reconnected with Madison and formed a new bond with Tinetti.

“I really do feel like everything happens the way that it's supposed to,” Madison says.