Is it permissible to demand the inheritance of the father and paternal uncle from their grandfather, who bequeathed his properties to his youngest son and daughter from another wife, and deprived the rest of his children of the inheritance?

If a grandfather registers his property to some heirs, thereby disinheriting others, this act is forbidden and void. This is based on the saying of the Prophet, peace and blessings be upon him: "Indeed, Allah has given every rightful person his right, so there is no will for an heir," unless the rest of the heirs agree. If the heirs permit it, it is considered their waiver of their right. However, if the grandfather genuinely sold the land for a known price, then the land belongs to the buyer. And if he gifted it and the donee took possession of it during the donor's lifetime, and the donor died before retracting the gift, then the gift stands. There is no harm in demanding the establishment of rights, and this is not considered a severing of kinship ties.