Brittny had never before seen or spoken to Killian, who lives in Italy and works at a wine and champagne company. "I've always known I have a half-brother, I was told as a kid, but he was totally off the radar. It was like he didn't exist," Brittny tells PEOPLE.
But Brittny's mom bumped into Nielsen at Ivana Trump's wedding last year, and they exchanged phone numbers – and, surprisingly, pleasantries. Then, last week, Nielsen "texted my mom and said Marcus is in town and he'd love to meet me," says Brittny.
So, Brittny and her mother met Nielsen and her husband Mattia Dessi, along with Marcus, for dinner at Il Pastaio in Beverly Hills.
"It was very weird and surreal," says Brittny, who hasn't been in touch with her father for several years. "I didn't cry. I'm too tough to cry. But we hugged, we talked about old stories. He said he watched Gastineau Girls in Italy, so he knew what I looked like."
"I asked him a million questions, but he doesn't speak English. He's really cute. He's like 6',5", blonde. He looks exactly like Brigitte."
Gastineau, who has a jewelry line, Tres Glam, with her mom, says it's surprising that Lisa was so supportive of the reunion. "It was very emotional for my mom," she says. "I mean, my dad left her for Brigitte, and she had a baby with him, but she was so supportive. She wanted me to meet Marcus."
Going forward, Brittny, whose BFF is Kim Kardashian, says, "It's kind of hard to know someone in just one dinner, so I'm definitely going to stay in touch with him. But I'm going to have to learn Italian!"