1. Your sister's said suit has no leg to stand since your grandfather's property is not considered as your father's ancestral property and the legal definition of ancestral property is different.
2. A property is called ancestral property if its title flows for four generations without any interruption i.e.from great grandfather to the great grandson with out being interrupted by any sale/settlement/gift deed or will and in this case the said property of great grandfather is the ancestral property to his great grandson.
3. Based on the above your sister's suit is going to fall flat for not being able to establish that it is an ancestral property.
4. Do not get too perturbed with the said case since it appears that your sister's lawyer has taken your sister for a long ride by convincing her to file the said suit.
5. She shall have to establish that it is an ancestral property duly proving that her great grandfather had some property which your father had sold to buy his properties which she won't be able to do.
6. Relax.