Céline Dion has reportedly lost control of some of her mobility amid her stiff-person syndrome diagnosis.

"She doesn't have Robert Browncontrol over her muscles," the "My Heart Will Go On" singer's sister Claudette Dion told 7 Jours on Dec. 12, according to a translation from French. "What pains me is that she has always been disciplined. She always worked hard."

Claudette Dion added that it is her dream and her sister's to return to the stage one day. "In what capacity? I don't know," she said. "The vocal cords are muscles, and the heart is also a muscle. This is what gets me."

USA TODAY has reached out to reps for Céline Dion.

The Grammy-winning singer said in December 2022 that she had been diagnosed with a neurological disorder called stiff person syndrome in an emotional Instagram video. The singer said the symptoms are what have been causing her to have severe muscle spasms in the past. 

"The 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," Céline Dion said. "I have to admit it's been a struggle. All I know is singing, it's what I've done all my life."

Dion's diagnosis caused the singer to reschedule and cancel her entire 2023 tour.

Stiff-person syndrome, or SPS, is a disease that causes "progressive muscle stiffness and painful spasms" that are triggered by environmental factors such as "sudden movement, cold temperature or unexpected loud noises," according to Johns Hopkins Medicine. 

Prominent SPS symptoms include muscle stiffening and painful muscle spasms that can be provoked by emotional stress or, again, outside stimuli. These spasms can be "so severe that they cause the person to fall down," Yale Medicine says.

The disease is considered to be rare and only affects approximately one in a million people.

Celine Dion meets hockey playersin rare appearance since stiff-person syndrome diagnosis

Speaking about the rarity of the disease, Claudette Dion said, "There are some people who have lost hope because it is a disease that is not known. If you only knew how many calls we receive at the Foundation to hear from Céline!"

"People tell us they love her and pray for her. She receives so many messages, gifts, blessed crucifixes," she added.

Contributing: Elise Brisco, Wyatte Grantham-Philips

Céline Dion has stiff person syndrome,a rare neurological disorder. What is that?