It is hard to make the argument of mental incompetence and it may get expensive -- lawyers charge by the hour for this type of case, and if you end up in a genuine will contest in court with a trial you could be looking at upwards of 15 grand in legal fees. If your grandmother's estate is worth that much that it would cover the fees for such a court case, then you might want to think about it. Other than that, there are no other real actions that you can take.
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