Can I remove a co-executor from my father's will if she does not wish to serve?

You can't remove an executor from your father's Will, but if an appointed co-executor does not want to serve, then she doesn't have to. Power of attorney is not required. Ideally, the appointed co-executor who wants to decline should sign a statement saying that she does not want to accept the appointment and stating some reason for that (too much of a burden, poor health, etc). If you are the other appointed co-executor, you would then seek to be appointed as the sole executor and include that statement from the declining executor with the Petition to Probate Will that you submit to the probate court. You don't have to have an attorney to help with any of it, but it is very, very, very strongly recommended.