Sister is our mother's Durable POA. Can she be petitioned because she is abusing her rights as an agent?

Your Mother lives alone? She takes care of herself? Then Sister is not there all the time? Why not enjoy a nice visit with your Mom when Sister isn't there? If your Mother lives alone she clearly doesn't have a severe dementia. If she wishes to change her POA she can certainly do so. There are "rights to petition" a POA. A POA is conferred on someone at the request of a competent adult.
When you go to your Mothers home, where she lives alone taking care of herself, and you knock on your Mother's door, what happens?
Your Mother, if she lives alone, likely gets her own mail? Or is she physically unable to collect mail?
Your Mother, if she lives alone, clearly has a phone in her possession. Why not give her a call and have a visit with her?
I can't help thinking there are things we don't know as regards this post. Can you fill us in a bit more?

If your mom is still competent she can change her POA, see who she chooses and talk to who she wants to. The POA is only for when mom is unable to make her own decisions. There are POAs that allow the agent to take action immediately upon assignment. It depends on the wording of the POA and your mom’s mental status.

This posting is a bit confusing to me.

Can you clarify please.