What are my options for gaining access to information concerning my moms estate as well as changing POA for aunt?

If your mother had any assets remaining after death her estate needs to go through probate using the laws of the state you live in when someone dies without a will. Probating an estate with no will can be a lengthy, complicated process with the state taking a chunk of the proceeds. An executor of the will would be named by a judge. If none of this has happened, you need to get an attorney. Your sisters authority as POA ended with your mothers death and she has no authority to handle the estate unless legally appointed. However - no of this has anything really to do with actions she took as POA while your mother was alive and your sister is not obligated to show you an accounting of financial matters that took place under her POA authority. The only exception would be if there was instructions in the POA agreement to be accountable to you or anyone else - and this is rarely written into POA agreements. As for your aunt - as long as she was mentally competent when she gave your sister POA - there isn't anything you can do - unless you have proof your sister is abusing her position - and then you can report her to Adult Protective Services and/or the police.