You can force her hand.  By that I mean that you can go down to the Probate Court in the County in which your Aunt resided at the time of her death and petition to be appointed as the Personal Representative of her Intestate (i.e., without a Will) estate.  If in fact there is a valid Last Will and Testament that your sister has in her possession she will have to come forth with it.  I am guessing that there is not.  Be aware, too, that once you are appointed as the Personal Representative you will have the power to question any dissipation of your Great Aunt's estate - her giving away the stuff counts here - and have the ability to take action.  Good luck.