When Brittany Oliver, then 29, looked down at her left hand on a beautiful day, she felt like she’d been punched in the stomach: Her diamond wedding rings were gone.

Incredibly, she would get them back the next day — and that would lead her to a new discovery: a long-lost sister.

It would take four years, though, for their story to unfold.

The saga begins on Labor Day 2015, at New York’s Seneca Lake.

“I’d been playing volleyball in the sunshine with my husband, Steven, and daughters when I saw the rings were gone,” Oliver, now 33, tells The Post.

Frantic, she retraced her steps. She even had friends, who owned a boat, post a sign about the missing jewels and cruise around the lake. But hours passed to no avail.

Eventually, she says, “We went home. I was devastated.”

While Oliver was combing the ground for her precious jewelry, Kala Rounds, another local spending her holiday at the lake, caught wind of her plight.

“I heard that a woman was in tears over a lost ring and jumped to help,” says Rounds, then 25. “I thought about the time I’d lost a bracelet from a boyfriend and how upset I’d been. I could only imagine how losing a wedding ring would feel.” So she joined a group of strangers determined to help her find them.

The search party spent hours scouring the shoreline. Rounds, now 28, remembers wading through waist-deep water, squinting through the muck on the lake bed, hoping to see a glint. Occasionally, someone would spot one and fish out a piece of jewelry — but there was no sign of Oliver’s diamond-studded wedding band or her engagement ring. They were starting to give up hope when a guy in the group hollered that he’d found the rings in the water.

Someone called Oliver, who hopped on her friends’ boat, while Rounds corralled the search team onto her family’s boat. They met in the middle of the lake for the handoff.

Both women remember seeing each other that day: the mother of two in happy tears, and Rounds, a disability-access counselor, pleased to have helped. The two went their separate ways: Oliver to her home in Watkins Glen, NY, and Rounds to Odessa, NY — both small towns, just 15 minutes apart. After an upbeat Facebook post from Oliver, life went back to normal — and remained so, until Christmas 2018.

“[My husband], Steven, thought a 23andMe test would be fun,” Oliver says. So he wrapped one up for his wife, who was glad to receive it. Having grown up with her maternal grandparents, a mother who was in and out of her life and an estranged father, Oliver was interested in learning about her ancestry and family.

But when the results arrived in January, something wasn’t right.

“There were family connections on there that I didn’t understand,” says Oliver. Unable to reach her mom, she asked relatives if they could explain the results, but everyone was strangely evasive.

After a day of confusion, “I finally spoke to my mom . . . and that’s when she reluctantly dropped a bombshell — the man on my birth certificate may not actually be my father,” Oliver says. “I went into shock.”

It turns out her mom had been dating two people at the time of her conception. One was the man Oliver grew up believing was her father. The other was Mike Rounds.

“One quick Google, and there he was,” she says. “The next day I took a deep breath as his home phone rang, ready to launch into this speech I’d prepared. But thinking I was a telemarketer he said, ‘Make it quick.’ So I just blurted out, ‘I think I’m your daughter.’ ”

An hour later, Oliver, Rounds, and his wife, Beth, met at a local restaurant. It turned out the two families had been living just 15 minutes apart.

That’s when Kala Rounds — now living in Syracuse, NY, two hours away — got a text from Beth, her mom.

“She told me everything. And just like that, I had a sister.”

The news didn’t come as a total shock to Rounds: Her mom had once told her that her dad might have gotten an ex pregnant. But for Oliver, it was a bolt from the blue.

“There was so much for me to absorb,” she says. “For 32 years, I thought I knew who my parents were. Then suddenly I have a different dad, a brother and a sister. It made my head spin.”

‘Suddenly I have a different dad, a brother and a sister. It made my head spin.’

As soon as Oliver returned from meeting her biological dad and his wife, she reached out to Rounds on Facebook. “I nervously typed, ‘Hey, I’m your sister.’ I wanted her to realize that it was OK for us to talk.”

The messages began flying immediately, and Rounds — recognizing Oliver’s name from that fateful incident in 2015 — mentioned the rings. Brittany was blown away.

“When I realized it was Kala who’d helped find them, I couldn’t believe it. This woman who’d spent hours searching [to help] a perfect stranger, she was my sister. It was overwhelming,” she says.

Four days later, the two met at Kala’s grandmother’s house.

“I was supernervous and kept trying to make jokes,” Kala says. “But as soon as we started talking, it was like we’d known each other forever.”

Both sisters were amazed at how similar they were. “When I decide to do something, I do it right away, no hesitation,” Kala says. “I’d never met anyone like that before, until Brittany.”

Brittany agrees. “The nerves just fell away as I realized I was a blond-hair, blue-eyed version of Kala.”

Now the sisters text constantly, and get together for meals, campfires and bowling. Brittany’s still amazed at everything that happened to bring them together. That day by the lake she thought she’d lost something precious — only to find something even better.

“Meeting my sister was like a perfect piece of a puzzle falling into place.”