Celine Dion's Sister Claims Singer Has Lost Control Of Muscles In Stiff-Person Syndrome Battle

A devastating update on Celine Dion's debilitating battle with Stiff-Person Syndrome ... as her sister says she's lost control of her muscles.

The star's sister, Claudette Dion, revealed the shocking turn in her disease which causes painful muscle spasms, saying it would be a while before Celine returns to the stage.

She told 7 Jours it broke her heart 'cause her sister had always been disciplined and hard-working ... recalling their late mother telling Celine her habits would make her successful -- but that very success now appears threatened.

Despite the grim outlook, Claudette says it's still Celine's goal to return to the stage -- but in what capacity, she doesn't know.

Research into the neurological disorder is still ongoing ... but because it's so rare, it means progress is slow -- which Claudette can't wrap her head around ... adding that many people in the same boat have "lost hope" of a cure ever being found.

The update comes after she told HELLO! Canada in Sept that Celine was "doing everything" in her power to recover ... but her spasms were impossible to control.

The "All By Myself' singer announced she was stricken with the disease in a December 2022 video message ... forcing her to cancel a bunch of her 2023 concert dates.

3 ½ years after last being seen in public, Celine was spotted last month at a hockey game between the Vegas Golden Knights and her hometown Montreal Canadiens -- looking in good spirits despite her health woes.