She cannot have a power of attorney over your mother because powers of attorney become ineffective upon the death of the principal. However, she may have been appointed as personal representative of your mother’s estate. This should not have happened without your having received notice. But to be sure, you can check with your local probate court to see if an estate has been opened for your mother and if your aunt has been appointed.
Did My Aunt Actually Have POA Over My Late Mother?
