Mom (98) has been in Independent Living for 22 years. How do we know when it's time to insist that she move to Assisted Living?

She had a stroke and some TIAs six years ago but is mobile with a walker, almost totally incontinent, and has some word recall issues plus unreliable memory and judgment. She has fallen several times, once with a concussion. She wears an emergency pendant and has cleaning help twice a month. I spend a day with her every week doing financial chores, letter-writing, doctor's appointments, etc. and am her POA. My sister and I found an aide to help her several times a week, but she fired the aide because she didn't want to spend the money. She has tried to refuse the pendant because of the cost, but we laid down the law! She can go down to meals and feed herself, though slowly, and often with choking. She can take her pills regularly, and can dress herself, though it takes up to three hours to do so. She cannot do cleaning or laundry, hang up her clothes ("too heavy"), write (because of the stroke) or water her plants.She loves to read books and nap--rarely watches tv or listens to radio--and can have a fairly lively discussion on many topics if she's having a good day.People who know her only superficially think she's far better-functioning than she really is. We feel she needs a little more help than we can give. Sister lives in a distant state and visits twice a year or so for up to 10 days, but is still working. I live 40 minutes away. We are not in disagreement, but we wonder when and how we will know it's time for AL. Do we have to wait till an emergency occurs? Mom has sufficient money but is very thrifty, to put it nicely, and says she wants to leave her money to us. We tell her we'd rather see her safe and comfortable in her golden years.