"I didn't see it coming, but it's also understandable that it's hard when you never see someone," Brie Bella tells PEOPLE

Nikki Bella‘s broken engagement came as a shock to fans — and her twin sister Brie Bella.

Just weeks before their May 5 wedding, Nikki, 34, and her fellow professional wrestler beau John Cena ended their six-year-long relationship, which was a surprise to Brie.

Speaking with PEOPLE, Brie admitted that she did not foresee the formerly engaged couple’s breakup.

“I didn’t,” says Brie, 34, on Tuesday while discussing the importance of children’s vaccinations in celebration of World Immunization Week, co-hosted by Rotary International and Red Nose Day. “I just think how busy everyone’s schedule is, and unfortunately with John and Nicole, they are two people in their careers who are really dominating what they’re doing, and in doing that, it means you’re always on the road.”

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“He’s filming movies and she’s doing all these different events. She’s one minute bottling up vintage chard in Napa and the next second she’s in New York representing Birdie Bee,” Brie said about Nikki, with whom she shares wine line Belle Radici.

“I think to myself as business people, in the last decade we’ve always been this busy, so it wouldn’t cause a loneliness. We’re used to it,” Brie added. “So I didn’t see it coming, but it’s also understandable that it’s hard when you never see someone.”

An insider previously told PEOPLE that Nikki was the one who pulled the plug on their wedding, for which Brie would’ve been matron of honor.

“John acted more and more like he was doing her a favor by somehow conceding to go ahead and marry her,” the insider said. “[Nikki] doesn’t need a pity proposal, a pity wedding, a pity husband. She’s a phenomenal, tremendous woman.”

Their romance — specifically their discussions about marriage and kids — has been documented on the hit E! reality shows Total Divas and Total Bellas.

“I do want to be a mom. I feel like I’d be missing out,” Nikki told Cena, 41, in 2013 on Total Divas, to which he replied: “I’m going to choose my words carefully. I can’t give you that.”

WATCH: Brie Bella Says She’ll ‘Always Love John Cena Like a Brother’ After Twin Nikki’s Breakup

On Saturday, Nikki made her first public appearance since the breakup at the Nirvana Food and Wine Festival in Arizona, where she and Brie hosted a rosé tasting party in partnership with their wine line.

“She’s good, they call her fearless Nikki for a reason,” Brie told PEOPLE. “She always puts on that brave face. My sister has more strength than any other woman I know, especially Wonder Woman. She’s bigger than that.”

Brie also shared that fans and viewers will see what went wrong in Nikki and John’s relationship in the upcoming season of Total Bellas.

“I really had to take a step back and allow it and that was really hard on me because it’s my twin. I want to protect her from everything,” the mother of one said. “This has been the hardest season to film for me because all I want to do is protect her and keep everyone away, but this is what we do.”