If your mother is living, she can file a suit for partition and separate possession of her legitimate share out of her deceased father's property divided by metes and bounds.
If your mother is not living then her legal heirs can file the suit as suggested in the capacity of her successors.
You can obtain certified copy of the registration documents to proceed with the institution of the partition suit.
If they rely upon any Will, let them produce it before court which can be challenged on merits.