I understand from my previous question that if my step-father is unable to perform the duties as executor of my mother's will that I, as the successor, can. His niece is trying to change my power of attorney for my step-father to herself. If she does this, can she take care of the few minor things of my mother's will or can I still do this? The only things to be done is take my mother's name off back accounts and the house deed. There was no insurance.

No. It is extremely unlikely that the step-father's duties as a executor can be delegated by a POA.

... His niece is trying to change my power of attorney for my step-father to herself. ...

As discussed in the other thread, if your step-father is incompentent, she would not be able to have him do that, absent fraud of some sort.