They are as close as two sisters can be.

Emily and Caitlin Copeland, of Houston, Texas, are 18-year-old identical twins who share an easy laugh, a love of cooking and a bond few can ever imagine.

Emily and Caitlin were born conjoined, fused at the chest, liver and bile ducts. Their parents, Crystal and John Copeland, were thrilled to hear they were pregnant with twins until the doctors told them they were conjoined.

"Our minds went completely blank the minute she said conjoined twins, so that was a long, hard, horrible weekend," Crystal told ABC News.

"I think the biggest thing we had was a lack of information on the whole situation," her husband, John, added.

"There just wasn't anything good on the Internet," said Crystal. "You know, one baby survives and the other dies. That was the worst."

Doctors at Children's Memorial Hermann Hospital in Houston did everything to ensure a safe delivery of the hospital's first case of conjoined twins.

"We could not promise her anything," Dr. Paul Cook, Crystal's Ob/Gyn, explained. "And you know, 50 percent of conjoined twins end up as still births. And those that are born, there's only 25 percent survival. So there was going to be a rough track the rest of the way."

One important factor that kept the girls' case optimistic was the fact their hearts were not shared.

"Importantly, the heart was not shared," said Dr. Kevin Lally, surgeon-in-chief of the Children's Memorial Hermann Hospital. "So I didn't think that there was going to be anything that would prevent us from doing a separation."

Fast forward 18 years later, and the twins, who were successfully separated at 10-months-old, are more than thriving. They are co-valedictorians of their high school class at Lutheran High North in Houston, and are now both about to head off to college-Emily to the University of Houston to major in hotel management and Caitlin to Concordia University Texas in Austin to study education.

"It's the first time we're going to be separated permanently," said a laughing Caitlin. "And so … kind of freaking out a little bit."

"It's scary," agreed Emily. "If I start over-thinking it, I get really, really sad."

While their connection may be strong, their interests are different. Caitlin likes sports and the outdoors and Emily prefers to knit and stay inside. Caitlin likes meat while Emily prefers vegetables.

However, when they're together, "we both get real loud," said Emily.

The girls don't remember being physically conjoined. The only clue is the scar on their chests, but they feel the closeness every day.

"When people talk to us, they're like, 'You just seem so close,'" Emily explained. "And I think being conjoined, you can actually look a picture, like, 'Wow, they really were close.' But, it's true. The physical aspect of it is 100 percent true emotionally."

The Copeland family is not only passionate about their faith, but about helping others who find themselves in a similar situation.

"We want to use this blessing to be a blessing to others," said Emily. "I hope we can help so many people and just be, if anything, a story to read and smile [about] when everything looks bad."

I would say to other families, 'Just don't expect the worst,'" added Caitlin.