Agricultural land does not govern by the Hindu Succession Act. The provisions of land revenue law will apply in transfer of share in agricultural land. Non-agricultural lands or lands beyond the purview of revenue laws are governed by the Hindu Succession Act (HSA) if owner dies intestate. If owner of the land has prepared a testamentary Will then also the HSA will not apply.
Consent of sister
The provisions of Uttar Pradesh Revenue Code, 2006 will apply in your case. Section 108 of the code, provides that widow, son (male lineal descendants), unmarried daughter are legal heir of first place. Mother and father are the legal heir of second place. Married daughter comes at the third place of succession if no heir present in first and second place.
The owner, his wife and sons are the legal heirs. He is sixty two years old so he has no unmarried daughter. He is alive, thus, his wife and male lineal descendants have right and title in the agricultural land. His sisters have no right in the land. Therefore, no consent of sister necessary for purchasing the agricultural land.
You should ignore the fact that consent of sister is necessary for executing a valid sale deed. Consent of sons are necessary because they have interest and right in the property. However, he can sell the land for the benefits of family but you should take consent of his sons. You can make his sons witness of the attestation of sale deed. Their signature will infer that they have given their consent in sale of the land.