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.

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Getty 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. Here's hoping for Celine's health, happiness and return to the stage for the New Year.