Sometimes the choices are very hard. The chances of you being able to go to court against this woman's daughter and obtain medical authority over her treatment would be very slim. Also, you may find, as many do in elder care, that the somewhat confused patient will grasp other family members and beg to be taken out of the nursing facility, will tell terrible stories of how they have been neglected or mistreated. And then when it comes down to brass tacks, and you confront the daughter over the bed, fully prepared to take action to rescue your sister, the patient will say, "Oh, I'll just do whatever Susie wants me to."
Getting someone into a full service nursing facility is not easy. Getting them out and moving them to a private home is nearly impossible unless you have pretty much unlimited financial resources and or lots of cooperative family help. If they are at that level of needing care, you'd need a lot of high tech medical equipment at your home. You'd need around the clock in-home caregivers. It takes about 5 reliable, vetted people, so they can cover all shifts, help you with moving, bathing, feeding, changing and treating the patient and allow you to have a life of any kind. There will need to be therapists who need to come in to provide her with occupational and physical therapy. People who take in very compromised family members soon find they can do very little alone, especially if your sister cannot get up and down easily by herself.
If you are really concerned about your sister, you can be an advocate for her. She would be available for you to visit in normal times, perhaps at this time, to Zoom with, or talk on the phone with every day or visit through the window. When things improve, and visitors are allowed in nursing facilities again, you'll be able to be with her in person. Do not be surprised if she constantly begs to be taken home, or tells you how horrible things are there. If you hear something that gives you concern, discuss it with her daughter (principle caregiver). My aunties were very always vocal and willing to check on my mom's well being when she was in this situation, and we worked together after we got through the first tense (are you sure about doing this?) aspects of the placement. But they soon came to understand that when my mother said, "I want to go home! This is a terrible place! I hate it here! Get me out of here! I want to go home!" it was a very valid feeling. It just wasn't possible for those wishes to come true because of the stroke and the situation, but we all did what we could to keep her content. Prayers for you and your sister and this situation.