As she will not be using the car again, may be easier in the long run to just transfer ownership into your name.
This is easily done by completing new owner section of logbook and posting it to DVLA.

Would be a good idea to get verbal permission from your sister as she is power of attorney.

Do you need/ use the car? if yes, even more reason to transfer ownership to yourself.

If not, just sell it? would save maintenance costs ect.