Dear Brahmam
Please note that on the death of your grandfather, the property would devolve on his children equally and if any one of the childred is dead, his share in the property would devolve on his children in equal shares.
Persuming that the property is undivided, it is ancestral property, and assuming that your grand father was only the heir of your great grandfather, he had become sole owner of the property and he could dispose it accordingly, since he has also not transfered his right by what ever means, and died intestate, the property will devolve to his children and widow (your grandmother) is a class one heir to claim properties in equal proportion.
Given that your mother has equal share in the property and can institute proceedings in the Court for partition and other reliefs and also challenge the sale or claim a share in the sale proceed.
As suggested by Lakshmi you can try to settle the matter in the Loak Adalath, However First try to get an intrim order of stay of sale in your favour immediately.