Cystic Fibrosis Sisters Christina and Ali Take Judges' Breath Away on "America's Got Talent"

(CBS) Christina and Ali, two sisters with a deadly lung disease, took the judges' breath away Wednesday night on "America's Got Talent."

The girls, 13 and 20, have a genetic disorder called cystic fibrosis. They didn't know if they would live long enough to make it to the stage, let alone have the strength to sing. But sing they did - a Miley Cyrus song called "The Climb."  

"I love to sing," says Christina with a glowing smile. "You are in a totally different world."

"I want the judges to look beyond everything, says Ali. "And just see us for who we are."

And they did.

"We were told since we were little we would never be able to sing," Ali told the judges after her performance.

Judge Sharon Osbourne replied, "You've proven them wrong, haven't you."

Judge Howie Mandel, who stood for most of the performance, said, "I want for you anything you want in life."

About 30,000 Americans, and 70,000 people worldwide, are living with cystic fibrosis, caused by inheriting a flawed gene from each parent. The disease causes a sticky mucus to build up in the lungs, clogging them and leading to life-threatening infections. It also clogs the pancreas so the body can't properly digest food.

Only a few decades ago, children with CF seldom survived elementary school. Today, thanks to earlier diagnosis and improved treatments, 47 percent reach 18 or older. Most of the 400 deaths a year are among teenagers and young adults, according to the CF Foundation's registry.

As for Christina and Ali, their story is only beginning. By unanimous decision, the judges sent them on to Las Vegas, where they will compete to become "America's Got Talent's" next champions and hopefully, breathe a little easier.

"If we pursue our dreams, it will give us something to look forward to rather than dwell on our disease," says Ali. "Cystic fibrosis does not define us. We define ourselves."