Well, you certainly have a potential issue here and should retain counsel if you believe there could be someone exerting influence that is "undue." If the language in the will calls for "per stripes" distribution then it is very likely you would stand in your father's shoes as you believe. One would have to review the will to be sure that the will does not state his share lapses if he predeceases grandmom which would leave you out.
Regarding the issue with changing the will: If your grandmother lacks capacity she cannot change the will. This is a heavily fact sensitive determination but you would need counsel to assist you (a current POA cannot change her will for her). If this person attempts to influence her or forces a person without capacity to sign a new will, you could have an action for "tortious interference with expected inheritance" or something similar.
Hire an attorney. Good luck.
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If the Will has not been changed, you would be entitled to your father's share if the language in the Will does not provide otherwise. You should consult an estate planning / estate adminstration attorney. It appears that your uncle is operating under a durable power of attorney - - - but most DPOAs do not allow an attorney in fact (agent) change estate planning documents. I hope you have a copy of the original will.
I agree with the other answers and would add that if your grandmother has a springing power of attorney, meaning the agent can only act upon your grandmother's incapacity, and your uncle is exercising power under the POA, this indicates your grandmother would not have capacity to change her will. Most POAs require certification of incapacity by at least one physician. If you have access, look over her POA to see what it requires as far as establishing incapacity. However, like the other answers state, you should consult an attorney. You may want to pursue a guardianship and/or conservatorship. This involves a legal determination of incapacity, so once said determination is made your grandmother would not have legal capacity to execute a new estate plan.
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