Property settled by Brgy. Captain

Hello! I have a problem that I hope you can help me with. My grandparents bought a lot in QC wherein a vacant lot is located beside it. The owner of the vacant lot advised my grandfather to take care of the lot while he transfers to the States. He gave his blessings to put a garden and put concrete if he likes. After the owner of the lot left the Philippines he never made contact eversince.My grandfather followed his advice to put a garden and parking space in the lot but unfortunately my lolo passed away already. Two years ago I think a letter was given to my grandmother to pay the realty tax. She got scared so she went to the barangay captain to consult on her problem. The barangay captain told my grandmother that he will pay for the realty tax and advised her that--"habang natutlog ang pera tatayuan namin ng bahay." To my surprise when I visited her house this afternoon the brgy. captain was building a house already for his daughter. They also placed a separate gate. He told my lola that if she does not pay the realty tax the government will acquire the lot already including the house and lot that she owns. I am worried for my grandmother's safety because the brgy captaing can just allow anyone to go inside because he was also planning to put a basketball court also. I told his daughter to give me a copy of the receipts of the realty tax that he has paid. Does the brgy. captain have the right to build a house in the property even if my grandparents does not own the lot but took care of it for decades already? Just in case he pays the realty tax does he have the right to occupy the lot? I know though that I have to go the assesors office to inquire who owns the lot and who was able to pay the tax years ago. My dad told me that he was able to pay the realty tax and instructed the brgy captain that he is not allowed to enter into the property. Unfortunately, my dad passed away last Apr. 4, so I will be the one to defend my lola. I know that there is a law regarding illegal settlers that the property will be owned by the people who have settled in a land for 11 years if no one claims for it. The owners have not claimed for the land for so many years and we do not know where to find them. I hope you can help me on this.