1. It is not clear as to whether your grandfather has bequeathed his estate in favour of your Uncle and mother. The said document is required top be perused for properly advising you in this regard.
2. Unless your mother has received the payment, she is not liable to pay the capital gain tax. Moreover, in case of inherited property, no capital gain tax is required to be paid since there has been no capital invested and gain accrued from that investment.
3. The said GPA can be revoked/cancelled and such cancellation deed can be registered in any other sub-registrar's office.
4. If it is clear that your mother has been bequeathed with equal share of your late grandfather's properties, then your mother can file a partition suit for legally dividing and demarcating the said properties and then deal with her share of those properties by herself including paying the property and other taxes levied on the said properties.
5. If the title of the said properties of your grandfather, since deceased, jointly stands in the name of your mother and uncle, then your mother can execute a will bequeathing her share of the said properties in favour of any body she wishes to.
6. Land is not registered. The Deed of Conveyance through which the title of the land was transferred in favour of your mother can be registered.