It may not be grounds to oppose him being appointed as Executor, but you should definitely consult with an attorney about a limited appointment to invalidate the lease, and a possible Accounting.
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If you want the will to control and wish to bring property back into the estate, you can bring a proceeding in Surrogate's Court to invalidate the lease on the basis that it was improperly procured. You would require evidence that your father did not understand what he was signing or that your brother used fraud or undue influence. How likely you would be to succeed depends on the strength of the evidence. These are typically tough, expensive cases. Start by consulting with a probate litigation attorney - not the attorney for the estate.
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First, understand that estate litigation can become extremely expensive. Although it may be a difficult thing to do, a cost benefit analysis should be undertaken to see if the value of an unencumbered 1/3 of the farm is worth the legal fees you will incur. If it is, then contact a local litigation/probate lawyer for a consult. The key issues will be your father's mental state and whether there was undue influence.
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