granted land: can it be revoked?

In 1986, my parents were granted a piece of land through NHA. This was under President Cory Aquino's housing program. During the time, the house they live in is built by my grandmother although the land where the house was built was not my grandmother's. In 1986, a census was conducted in my parents' place. During the time, he was living with his mother, the owner of the house. When they were told that it's for the land granting program my father told the representative who was conducting the survey to just name it after her mother, however, the representative said that his mother is not eligible for that since she doesn't have any source of income and she is already over-aged. During the time, it was only my father who was working and eligible for that. After some time, the land title was given to him and my mother. They were able to pay for the land and the mortgage was canceled. Now, my father's siblings are fighting for the land claiming their rights over it. Also, they are harassing my parents to leave the land and even threatened to file a case against my parents. Now, The title of the land is under my parents' names. Can my father's siblings run after it? Does NHA have the right to revoke the land already granted and fully paid for? If so, under what circumstances can they do that? Thank you.