Based on the facts stated by you, the crucial question is whether your grandfather's Will had already taken effect upon his death and whether your father survived your grandfather.
From your narration, your grandfather executed a registered Will in favour of your father and thereafter passed away about five years ago. Your father was alive at the time of your grandfather's death and only passed away last year. In such circumstances, the bequest under the Will ordinarily vested in your father immediately upon the death of your grandfather. Mutation or revenue entry in the records is merely an administrative process and does not itself create or extinguish title.
Therefore, if the Will is valid and your father survived the testator (your grandfather), the property would ordinarily devolve upon your father under the Will from the date of your grandfather's death. The subsequent failure of the revenue authorities to complete mutation in your father's name due to administrative reasons does not normally alter the legal effect of the Will.
Consequently, upon your father's death, the property would form part of your father's estate and would devolve upon his legal heirs according to his own Will (if any) or, if he died intestate, according to the applicable law of succession.
On the facts stated by you, your father's brother would generally not acquire a share merely because the mutation was pending. Mutation entries are evidence of possession and revenue administration; they are not documents of title. If the property had already vested in your father by virtue of the Will, the pending mutation does not reopen succession in favour of the other children of your grandfather.
However, one important caveat remains. Since the property is described as patta land allotted to a landless farmer in Uttar Pradesh, it is necessary to examine the original allotment conditions. Certain categories of government-allotted agricultural land may contain restrictions on transfer, inheritance, or devolution. Therefore, the allotment order, patta documents, and applicable revenue laws should be examined to ensure that there is no special statutory restriction affecting succession under the Will.
Subject to there being no such restriction, the legal position is that the property vested in your father on the death of your grandfather, and thereafter it would devolve upon your father's legal heirs. Your father's brother would ordinarily have no independent claim merely because the revenue records were not updated before your father's death.
You should therefore preserve and produce:
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The registered Will of your grandfather.
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Death certificate of your grandfather.
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Death certificate of your father.
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The original patta/allotment documents.
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Any records showing that mutation proceedings were initiated but remained pending.
These documents will be important in establishing that the property had already vested in your father before his death and now forms part of his estate rather than the estate of your grandfather.