Sharing 59-year-old Kelly's diagnosis on Instagram, the actor urged her followers on social media to wear a mask and save their lives as well as of the people around them.
"My sister Kelly, who already has lupus, now has Covid-19. This is her hospital room. One of you non-mask wearers did this. She does not have an immune system. The only place she went (to) was the pharmacy.
"Can YOU FACE THIS ROOM ALONE? Wear a mask! For yourself and others. Please," Stone wrote alongside photographs of her sister's hospital room, featuring a health worker dressed in full PPE and medical equipments.
The 62-year-old actor, who also posted a video elaborating on her sister's condition, further revealed that Kelly's husband Bruce Singer was "fighting for his life".
Stone said that her mother, Dorothy, who has had two heart attacks and a pacemaker installed since lockdown, has been unable to get tested for the virus.
The actor stated that she also tried to contact governor of Montana, where the family lives, and hasn't been able to get in touch him.
"My grandmother died of Covid and my godmother died of Covid. My sister and her husband are fighting for their lives and my sister is not doing well."
Stone concluded her video by urging her followers to vote as she declared support for Democrat party leaders Joe Biden and Kamala Harris in their campaign for the US presidency. SHD RDS RDS