Alienable land dispute

Good day! My family owned a a lot with a commercial building for the last 15 years. The lot was bought by my late father from the previous owner but he has unsigned deed of sale but signed witness by the barangay captain. We failed to have it signed because the owner died before the deed of sale was made. We had the land checked in CENRO Office and certified that the lot is alienable and disposable.However, we failed to process transfering the land to his name. Now, recently, the grand children of the previous owners are claiming that they still own the land and they sold it to another owner and made a deed of sale. The allegedly new owner is now claiming that she owns now the property and we should demolished the building. My question is, who has the legal rights with the property provided that the land is alienable? Does the grand children has the legal rights to sell the property without informing the actual occupant? Does the grand children has the right to sell the property? And lastly, does the deed of absolute sale of the allegedly new owner is valid or legal? Thanks in advance