Las Vegas, Nevada - Céline Dion's sister has shared a troubling update on the star's health amid her battle against Stiff Person's Syndrome.

It has been known since the end of last year that Céline Dion is seriously ill.

The My Heart Will Go On performer suffers from Stiff Person's Syndrome, a rare nerve disease that causes severe muscle cramps, especially in the back and legs.

The 55-year-old has largely withdrawn from the public eye and turned her back on the stage for the time being due to her health issues, postponing her planned world tour again and again until she finally canceled it completely.

But even though the Canadian-born singer is fighting with all her might for a comeback, there seems to be little hope for this at the moment.

Céline has lost control of her muscles, said her older sister Claudette (75) in an interview with a Canadian TV station, according to the Daily Mail.

"She's working hard, but she doesn't have control over her muscles," Claudette revealed.

According to her, Céline's big goal is still to return to the stage at some point. When and in what form exactly? "I don't know," her sister admitted honestly.

"The vocal cords are muscles, and the heart is also a muscle," Claudette explained. "Because it's 1 out of a million case, the scientists haven't done that much research because it didn't affect that many people."

Stiff Person's Syndrome cannot be cured, but with the help of appropriate medication, the autoimmune disease should be kept under control.

However, this does not seem to work for Céline Dion. Despite working with "the top researchers in the field," her condition has hardly improved.

"We can't find any medicine that works, but having hope is important," Claudette shared in a previous interview.