My mom seems increasingly weak -- she's almost ninety. How safe is she living alone?

So far, I've been able to help my Mom in all the ways she needs help. But yesterday she fell in her bedroom and couldn't get up. The rest of the day, although we visited the doctor, she seemed weaker. I have two sisters, one of whom seems to bring trouble wherever she goes. The other one and I get along extremely well. I now feel at the point that Mom is not really safe living at home, and she has quite a bit of tiled flooring. She does live mostly on the main level, and has stair chairs to the other floors. We take dinners together almost every night, but that may soon be impossible. I already feel as if I've given a large portion of my life up to care for her. I don't seem to know where to draw the line, and I'm apprehensive about discussing future options with both siblings. Does anyone have any advice about these issues -- Mom living alone and sibling agreement on what to do about Mom's care? I would so appreciate hearing from you. I find this Web site so very helpful during this challenging phase of my life. My mom and I are both widows...