Céline Dion has “lost control of her muscles” as she keeps enduring stiff person syndrome. The ‘My Heart Will Go On’ singer, 55, revealed her diagnosis of the condition in December 2022, which forced her to pull the plug on her ‘Courage World Tour’, and her sister Claudette Dion has now shared a distressing update on her sibling’s health battle. Claudette, 74, told Canada’s 7 Jours about how Céline has “lost control of her muscles”: “What breaks my heart is that she’s always been disciplined. “She’s always worked hard. Our mother always told her, ‘You’re going to do it well. You’re going to do…