Celine Dion health: Singer's sister says Grammy winner 'doesn't have control over her muscles'

LAS VEGAS -- New details are surfacing about the condition that's taken Celine Dion off the stage.

The multi-Grammy-winning legend has been fighting for control over her own body for more than a year, ABC News reported.

And now, while she still "dreams" of returning to the stage, her prognosis is not looking good.

Dion's sister is giving an update on the singer's neurological disorder.

Claudette Dion said her sister is "working hard, but she doesn't have control over her muscles. What breaks my heart is that she's always been disciplined. She's always worked hard."

Dion previously said she was diagnosed with a very rare neurological disorder called stiff person syndrome, which affects roughly one in a million people.

The five-time Grammy winner announced last year the diagnosis forced her to cancel her world tour.

SEE MORE: Celine Dion reveals she's been diagnosed with rare neurological disorder, reschedules tour

"These spasms affect every aspect of my daily life sometimes causing difficulties when I walk and not allowing me to use my vocal cords to sing the way I'm used to," Celine Dion said.

The condition is progressive and incurable.

It causes the body to attack its own nerve cells, and can affect mobility, vision and speech.

In October, Dion made her first public appearance in more than three years, attending a hockey game with her sons, where she met with players and reportedly sang "a few notes."

Her sister said Dion hopes to perform again one day.

"The idea is to return to the stage. In which state? I do not know. The vocal cords are muscles, and the heart is also a muscle? We can't find any medicine that works, but having hope is important," she said.

Her sister said research into the condition is ongoing, but because of the rarity of stiff person syndrome, progress has been slow.