Can step-grandmother legally receive great-grandfather’s land by trans

My great-grandfather bought land in his own name. After his death, my grandfather registered the land in his name. My grandmother passed away due to a health issue, and later my grandfather married another woman. He then transferred some of my great-grandfather’s land to my step-grandmother’s name. Now they are not giving any land to my father and are trying to transfer it to my step-grandmother’s son. Is it legal to transfer my great-grandfather’s land to my grandfather’s second wife? If not, how can we proceed to reclaim the land?