You need to pursue this as a case of Elder Abuse! The NCEA (National Center on Elder Abuse) website should help you find local resources to help you and advise
you how to proceed- http://www.ncea.aoa.gov/ncearoot/Main_Site/index.aspx
If you haven't already, immediately start a record/file to document every detail of this situation! Your perceptions and observations are obviously very important, but they carry much more weight when another party can validate and/or support them to a judge. Document everything your grandmother says to you and be sure to note the emotions that she projects (or even that you perceive) when she says anything- such as 'fear' or 'desperation' in pleading for you to get her away from your aunt.
Any visible signs of physical abuse or neglect should be photographed immediately and authorities should be notified. Financial abuse is the most common sort of Elder Abuse and it's usually the easiest to prove...if/when you gain access to financial records! If you are truly concerned that the financial abuse (aka- theft) is happening, you could always try to get the bank to send copies of previous years bank statement detail for your review...if you pretend to be 'her'....