We have always called them “Guilin sisters,” these seven girls adopted the same day from the same orphanage in Guilin, China.

They are scattered across the country, so we try to schedule annual reunions. “It’s important because this connection with girls from the same orphanage may be the closest they ever get to biological family,” explained Lynne Craine of Elkton, Md.

After all, the parents feel forever bonded after two exhilarating weeks together 10 years ago when we traveled to China to adopt our children. We called each other our “travel group,” as if we had all booked the same cruise, but it was so much more than that. It was a bond as instant and intense as anything I’ve experienced — part world tour, part maternity ward. Picture the most exotic travel junket of your life, from which you happen to bring home a new baby.

Every minute of that trip remains vivid as a movie for me, but most of the girls have no memory of their birth country or of their hometown, Guilin, famous for its stunning jagged mountains. Only one of the families has made the pilgrimage back to China.

The seven families are spread across the country in California, Colorado, Maryland, New Jersey, Missouri and Indiana. Despite the distance, we do our best to get the girls together as often as we can. After all, the girls were sisters in the orphanage, and they are each others’ link to China.” They share a history that’s impossible to replicate. Ni-Ni was 10 days old when she was found at the gates of the Guilin Social Welfare Institute. The other girls had similar histories. They slept in six neat rows of cribs in the same airy, sun-lit nursery.

As the girls reached their preteen years, however, we could no longer count on an instant connection. “They have seen each other in the past, but I think that this vacation established a connection among them that is independent of the parents saying, ‘You will see the Guilin girls,’” Lynne observed. “They are old enough to make their own friends.”

And they are old enough to act resentful and bored if we team them up with kids who aren’t to their liking. Even with only three families attending this reunion in the supremely kid-friendly Wisconsin Dells, it could be a very long week if they didn’t get along.

I should have taken NiNi’s word for it that I didn’t need to worry. “We’re all really close, and I like to maintain a good friendship,” she said. “If you only see each other every ten years, it would be all awkward.”

They carried on like sisters, minus the sibling rivalry and the friction of day-to-day living. On this trip, we noticed a growing resemblance between NiNi and Evie DeLong of Indianapolis, who’s a few months younger, that was so striking they could be mistaken for sisters.

“This vacation could have brought conflict and jealousy, but that did not happen,” Lynne said. “They truly liked being with each other.”

Within minutes, it seemed, they were scheming sleepovers and trips to the Mt. Olympus amusement park and Jeremy Allen’s Illusions show — all part of the dizzying entertainment strip that makes The Dells a sort of G-rated Vegas. There are plenty of attractions for the grownups, too, from the beautiful lakeside scenery to a day trip to Frank Lloyd Wright’s home, Taliesin.

Yet it is the quiet moments I’ll remember most from the trip — the girls making crafts with colored duct tape or taking goofy pictures with the iPad. Staying up late and laughing like they’ve known each other all their lives.

Like most adopted kids, these girls vary widely in their degree of interest in their cultural heritage and their birth families. But there’s a distinct difference from many domestic adoptions today, where the children get to know their birth families or have some information about them. There is little chance of that for these girls, even in the distant future. Some days, they can’t help but wonder, can’t help but feel that loss.

How do you explain the chemistry that made this reunion work so well? Perhaps these girls are as naturally compatible with each other as we are with their fabulous parents. Yet throughout our week in The Dells, I couldn’t help but think that this was something better, something deeper, than a relaxing vacation.

As usual, it was the kids who put it into perspective.

“We’re sisters,” NiNi explained.