Is it permissible to withhold a portion of the inheritance to give it as charity on behalf of the deceased, considering that the heir had renounced their share then claimed it after the death of the inheritor?

If the grandmother, while in good health and of her own free will, relinquished her right to your father's inheritance in favor of your brother, this is a permissible and enforceable gift (hiba) provided your brother took possession of it during her lifetime. If he did not take possession of it until after her death, then the gift is not enforceable, and the right remains her property and passes to her heirs. If the gift is valid and enforceable, your uncle has no right to claim it, and your brother is permitted to dispose of it. However, if the gift is not valid, then the money is your uncle's right, and nothing from it should be concealed.