Thanks everyone for the words of support and the sharing of experiences. Tybee, I will check to see what type of POA I have. I think the input on this thread will give me the impetus to cull at least another bag of clothing while she is staying at her friend's house. She has some items that are dated or otherwise unflattering that should go, especially since she has so many beautiful things that look really good on her. She is actually grateful if I tell her something is unflattering, so I'll try to think of this as a little bit of proactive vetting.
The best thing is that sister and I are best friends and on the same page about Mom, and we share the same values and ideas on end of life care as Mom. Sister is an RN, and she says she will move in with Mom when that becomes necessary, but we just keep playing things by ear as we go along. Any decluttering we do now will certainly be helpful if sister does end up moving in.