1. The quetion of the death certificate of your maternal grandfather is not all together so important.
2. If the copy of the pahanis is available with your mother then you should verify from its contents that whether upon the death of your mothers grandfather the names of the legal heirs ( i.e. your maternal uncle, and all other six sister's and the name of your mother's mother) have been incorporated in such pahanis i.e extract of the record of rights of such land in context.
3. If still only the name of your mother's grandfather is shown in this pahanis then without the mutation of the names of the legal heirs of your mother's grandfather, your mother's brother cannot sell such land in context. And if he has done so then it is illegal and hence though he has sold the land without your mother and her sister's consent then the entry of such sale deed would not be incorporated in the record of rights by the vilage revenue officer (Patwari) since the mutations of the names of the legal heirs of your mother's grandfather have not been effecuated in the pahanis. Therefore he is now forcing your mother and other sisters of your mother to sig some documents , which your mother should not do.
4. Even the name of your mother's father should have been incorporated after the death of your mother's grandfather. But it appears that your Nanaji's name has also not been mentioned in the pahanis.
5. It also appears that your maternal uncle and your mother and her sisters were not residing in the village of your mother's grandfather and so the mutations must have not been carried out.
6. What your mother and her sister together should do is to file a civil suit under order 20 Rule 18 of Civil Procedure Code, 1908 seeking a decree of partition of such property in context and and separate possession of share by making your mother's brother as respondent.
7. The death of your mother's garndfather and your mother's father can be vouched by any relatives of your mother by maqking an affidavit. This would suffice for to adduce evidence of the ddeath of your mother's father and her grandfather which would be needed to have a legal heirship certficate attested by an Executive Magistrate and then based upon which the court will be able to decree the partition and put the living legal heirs of your mother's grandfather into possession of their respective share as per Hindu Succession Act, 1956.