What do people do for back up caregivers?

My mother is in a wheelchair, virtually blind, with dementia. My father is difficult, wedded to his ways, his house, his refusal to change his methods or habits. They have a saint of a caregiver who is there weekdays 9 - 3 and weekends 9 - 12, comes back at 3 on weekends to help end naps and also at 6 to help put mom to bed. They live in coastal SC. This will be the 4th year my sister puts her NC life on hold to come evacuate with them. After this one, she's done. I'm too far away. New people don't know his routine; he gets nervous, yells at them, yells at mom for not cooperating, etc. and causes all sorts of problems.

What do others do for back-up caregivers? How does one find a caregiver who is technically on standby and knows the routine, especially for evacs? He's not going to move and many of the agency people won't return.