From the facts stated by you, the crucial issue is whether your father had the legal authority to cancel the original registered sale deed executed in favour of his minor grandson and thereafter execute a fresh sale deed in your favour.
If your father had executed a valid registered sale deed conveying the 5 acres to your nephew (your elder brother's son), with your father acting only as the natural guardian accepting the property on behalf of the minor, then ownership vested in the minor immediately upon registration. The fact that the beneficiary was a minor does not make the transfer revocable.
In such circumstances, your father could not unilaterally cancel the sale deed merely because he later changed his mind. Once title had passed, the property no longer belonged to him. Any subsequent cancellation deed or mutation obtained before the revenue authorities, without the intervention of a competent civil court, would ordinarily not divest the minor of his title. Revenue authorities cannot adjudicate title disputes or annul a registered conveyance.
Further, any sale of a minor's immovable property generally requires compliance with the provisions of the Hindu Minority and Guardianship Act, 1956, including obtaining permission from the competent Court where such permission is legally required. A transfer made in violation of the statutory safeguards is liable to be challenged by the minor upon attaining majority.
The affidavit executed by your father expressing his intention to give the land to you may support his personal intention, but an affidavit cannot override or cancel an earlier registered sale deed. Title to immovable property is transferred by a valid conveyance and cannot ordinarily be altered by a subsequent declaration of intention.
Since you have stated that your nephew has already succeeded in the civil suit and the revenue records have been mutated in his favour, the position is presently governed by the decree passed by the competent civil court. If that decree has attained finality, your remedy would ordinarily lie in filing an appeal before the appropriate appellate court, if you are still within limitation, or by seeking condonation of delay if sufficient cause exists.
Your continued physical possession of the land is a relevant fact and may have consequences in any future proceedings relating to possession or execution, but mere possession by itself does not confer ownership when a competent civil court has already declared title in favour of another person.
Therefore, you should immediately obtain and have an advocate examine:
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the original registered sale deed executed in favour of the minor;
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the document by which your father purported to cancel that sale deed;
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the subsequent sale deed executed in your favour;
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the judgment and decree passed in the civil suit; and
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whether any appeal has already been filed or whether the limitation period for filing an appeal can still be invoked.
Without examining these documents, it would not be possible to advise whether the decree can still be challenged. However, based on the facts narrated, the unilateral cancellation of the original sale deed in favour of the minor is likely to be the principal legal difficulty in your case.