Land transfer

My grandfather had died several years ago without doing of his land property. H has only three sons and one of them is my father.No legal act had been done to transfer the ownershp of my grandfather's property before or after his death.No registration and mutation has been done still now in favour of three of his sons. My father had left us(my mother,my sister,and me) many years ago doing second marriage but no divorse had been done and legal step had been taken by my mother because of financial support.Now I am established, my sister is married and my mom has been leaving with me in our ancestral house and have the possession total of his property during past 10 years and above. My father now came suddenly and demanded that he has sold all his property and land to a stranger already and also demanded for oral demarkation so that he can help the byer to have the possession a part of his property! Can he do that? Can he sell the property without anyone's consent? Can he make me and my mom shelterless? have we any right to defend this illegal transfer? what should we do now? please share your opinion as soon as possible!