Can a power of attorney document contain a clause that annuls original will?

Please accept my condolences on your loss. Under Georgia law, a person can revoke her Will either (1) by destroying the original Will with the intent being to revoke it, or (2) by executing a new Will that revokes the prior Will. A Power of Attorney is not a Will, so on the face of what you present it does not look like the provision in the POA should have successfully either revoked or modified your grandmother's Will. If for some reason the POA does manage to qualify as a Will, it may have successfully revoked the Will; however, that would be very unusual. A POA agent cannot change or revoke the Will of the person who made the POA, either. As for contesting the POA document, you really can't do that unless it is offered for probate as a Will. However, you may well be able to offer the prior Will for probate and, if needed, to compel your uncle or whoever else has it to offer it for probate. This is not a do-it-yourself task, however, Consult a good estate and trust litigation attorney (and please note, I do NOT do litigation) and have that attorney sit down with you, review as much as you can provide about the facts of the situation, and help you figure out what options you may have and what the likely cost of pursuing them may be. Best wishes to you.