Can we assign a temporary POA?

I checked with one of the nurses at dad’s and if surgery is necessary my sis will be contact until I’m able to take calls.

If she's already designated as your back-up she can take over if you resign. If you and she both agree that this should be a reversible, pro tem arrangement over the surgery and recovery period, I don't know but I imagine that shouldn't be impossible - can you check with whoever drafted the DPOA originally?

She is listed if I do not want to be POA. I’m not worried about the couple of bills but if he becomes near death. I’m of course planning for something that may be a non issue.

POAs can't be transferred. Does your POA list your sister as a back up? If it does, that is the only way she could do it while you are in the hospital. You could have your sister bring you the bills and check book for you to pay.