We can't afford full time care for mom.

Mom is 90 and lives alone in her house. I see her twice a week for about 8 hours. My sister sees her approx. 10 hours a week. We have help coming in for an additional 12 hours a week. We applied for VA benefits, but only got $143.00 per month assistance. My sister and I have to pick up the balance. Mom needs care 24/7 but we cannot afford the expense of around the clock care nor can we afford assisted living. Mom has an alert button she wears around her neck but I still don't feel comfortable leaving her alone. What can we do?.