When my elderly grandfather was very unwell, he came to live with us. I was a teen at the time. Nobody knew how long he had left, months or years, including my parents. It was an open-ended commitment.
Until you’ve experienced it, you can’t know how much having an elderly person living in your home changes everything. Childcare. Mealtimes. Holidays. Having people over. Going out. Keeping a pet. Not to mention finances, or wider family dynamics.
As such, I think you should hold your counsel about the ins and outs of what’s going on with your mum. She’s been there 7 years. I’d your sister has children, that’s a heck of a long time, a lot of compromises, many sacrifices.
You say that you want and need nothing from your mum. Good. Leave it at that. Visit her. Spend time with her wherever she’s living. And nothing else.
If you don’t like the arrangements your sister is making, have your mum come to live with you. If you can’t - keep quiet. She’s being looked after, and well enough by the sound of things. That’s enough.