Can grandfather’s relatives claim property after father’s legal inheritance transfer?

My father lived with a family member (my grandfather) who had no children. My grandfather passed away in 1978 without leaving a will. He had purchased property but did not assign it to anyone. Later, my grandmother legally adopted my father and, in 1980, executed a registered will/power of attorney transferring all her and her husband’s property to him. Since then, my father and our family have been living and running a business on that property. Now, my grandfather’s relatives are claiming ownership of the property as legal heirs.