What is the ruling on treating a stepfather with piety and the like? And is the questioner sinful if he leaves the house and disappears, knowing that his real father is his paternal uncle, and that his family deals with jinn, and some of them practice sorcery and witchcraft?

Lineage is determined by the legal father, who is the mother's husband. It is not permissible to claim another lineage unless there is a valid Shariah-compliant reason. A severe warning has been issued against anyone who knowingly claims lineage to other than their father. The fundamental principle is that the father is the known head of the lineage and has a right to dutifulness and kindness, whereas an uncle is not like a father in the obligation of dutifulness. There is no custody over a mature, sane adult, and such an individual has the choice to reside with whichever of their parents they wish, or to live independently, while continuing to show dutifulness to both. Hiding oneself incurs sin if it leads to a severance of ties.