Second wife children rights for ancestral property

My grandfather has a self acquired property he didn't will the property before his death, that had been partitioned among my father and his brothers by orally, my father got 4.2acers of agriculture land and he didn't earn any self acquired property. My father married my mother without divorceing the first wife.my step mother has 2sons and im the only one to my mother. my father never bothered about me and my studies or my future plans,however my mother worked hard and supported me for my studies, he never supported us finacially now my father along with my step brothers sold the entire land without my knowledge 6months back and he stopped coming to our home. do i have the right to claim my share in the property. we had sent the lawyer notice regarding this to my father he replied to the notice saying that he never married my mother. he was just having extramarital affair with my mother I'm just his biological son which is I'm a illegitimate son of him so i don't have any right for asking my share in ancestral property. i never thought this day would come in my life where my father denying the relation of us. I'm mentally very disturbed my world became upside down by hearing the words my step brothers calling me bastard. I'm presently working and taking care of my mother. we have all proofs of marriage, my birth certificate, Aadhar card, family ration card. i never thought my father would deny us. please give me the direction of hope by which i can remove the remark created as bastard and claim my rights. one of the marriage match recently got cancelled after they learnt this. please help me.