Am I sinful for not asking about my father who disowned me since birth due to family problems, and is it my duty to ask about him despite his disowning me?

The issue of establishing or denying paternity requires a ruling from the Sharia court. The father who knows he is your father must fear Allah and acknowledge you as his son, repenting for his denial of you. Indeed, denying a child is forbidden due to the severing of established lineage and the forfeiture of rights that result from it. The child must be dutiful to his father. And the child must not claim lineage to someone he knows is not his father, for it is authentically reported from the Prophet, peace and blessings be upon him: "Whoever claims lineage to other than his father, knowing that he is not his father, then Paradise is forbidden for him."