Power of attorney

Hi, My great grandma had given power of attorney to her maternal relative just because he was very cunning and smart, so this was back in 2008, now last year 2017 september ...she died and in the year 2014 she wrote a will for us for all the assets which she had, as we are the only legal heir . Now my question is can I notify to the competent authority about it and inform them that the owner is no more and the said power of attorney is revoked and anything done by the POA holder will be illegal. If yes than whom to contact and where to contact and what will be the cost. Please help me in this regard as I am naive in legal matters. Mumbai