Do I need to get another POA because my sister is delusional and is unable to make decisions in her best interest?

You don't need another medical POA, and if your sister is unable to make decisions she now can't give you any other sort. What decisions need to be made for her?

In order to get a POA, the person granting POA must be of sound mind and able to make the decision herself.

It sounds like it is too late for that. I think what you want is guardianship which is a court order. If you only have a medical POA, you will likely be unable to do her banking for her or make any other financial decision for her. (I say unlikely because my father's checks were accepted with my signature and no questions and I didn't have POA but more complicated financial matters will require guardianship or POA))