I am sorry for you situation.  If your Mother was not competent to sign the power of attorney then you can make an application to the court to have it made void (revoked is the proper term).  What you need to do is to go to court to be appointed as your Mother's guardian or conservator. At that point in time you can make sure that the POA is revoked (which it will be by law most likely).  Once that is done you will be able to take control of the situation and make sure that your Mother is cared for both physically and financially.  But be prepared for a battle from your sister as it seems that she wants to control everything.  Get help from an attorney in your area.  Good luck.