Is it permissible to claim a grandfather's inheritance twenty years after his death, especially if some of the heirs at that time did not pray, and some of them died, while being among those who prayed, and others continued to live in the grandfather's house, knowing that the father and his deceased brothers did not forgive anyone, and that the remaining brothers are not full siblings?

Rights do not lapse with the passage of time. Therefore, if your father and his brothers did not receive their share of their father's inheritance, and there is no proof of their relinquishment of it, then you may claim your father's share of his grandfather's inheritance. Similarly, the heirs of your deceased uncles have the right to claim their ancestors' share of their father's inheritance. The jurists have agreed that a right does not lapse with the passage of time, and the majority of jurists have not differentiated between old and new claims when hearing cases. The Sharia courts are the ones that decide on these matters.