Is it permissible to demand the rights of a deceased father who verbally relinquished his share to his sisters?

Your father's relinquishment of his right to inherit from his grandfather is a valid donation if he was in good health and the legal possession took place. This act of his is a righteous deed for which he will be rewarded. It would have been more appropriate for him to leave you sufficient wealth, acting upon the words of the Messenger of Allah, peace and blessings be upon him: "It is better for you to leave your heirs wealthy than to leave them poor, begging from people." What your paternal uncles did is contrary to maintaining ties of kinship, and they should have reciprocated your father's kindness. You do not have the right to reclaim what your father gifted to his sisters if the gift met its conditions. We advise you to be dutiful to your paternal uncles, speak to them gently, and seek the mediation of righteous people for reconciliation.