I would feel guilty going out to dinner or movie alone with my husband or having lunch w/ a girlfriend and not including her. She would feel "left out" and get depressed. I would like her to go to an assisted living nearby where I could visit her often. I have a sister and brother who can't take mom for various reasons. When I put myself in mom's place, I am reminded that I will be there sometime and my kids may not want me either. (even though I am more resourceful than mom, who has no hobbies and doesn't read.)
My mother, husband and I had a frank talk about this when Mom moved-in. We stated that we were all used to living our own lives, now we're thrown together, and that it's going to be hard to be together so much and that we probably need to force time apart to make it work. Our thought was that, otherwise, we'd end up getting on each other's nerves and we were all worried about what would happen if we couldn't manage to all live under one roof.
If you can't see having this talk between all of you or are certain the outcome would not end up with a constructive outcome, then I think you're going to be miserable with her there, quite honestly. And, if you're miserable, I think she'll realize you're miserable and will also be miserable knowing that she's causing such misery.
For instance, diapers. She just now started needing them as her knees are giving out on her. My mother is very proud and this was devastating news that she has to be reminded of. Hopefully her knees will heal with some rest. BUT, your feelings very gently delivered with your sister and her understanding perhaps even with her decision would leave you no worse off than you are now. But your mother may surprize you. I had to put my mother in a home for a year and she is the most unsocial woman in the world. It turned out to be the best thing for her. I pray that these examples will help you in your family's decision making. The writer above has it right. It is a very personal decision but it should include the whole family including your mother. God Bless!