Is it permissible for a brother to disown his family, claiming injustice, even though he received a share from his parents and relinquished the rest, and claims a fatwa from an Imam supports his position?

Severing ties of kinship is a grave sin and a destructive major offense. This is indicated by the words of Allah Almighty: "So, would you, perhaps, if you turned away, cause corruption in the land and sever your ties of kinship? Those are the ones whom Allah has cursed, so He made them deaf and blinded their vision." And the words of the Prophet, peace and blessings be upon him: "The one who severs ties of kinship will not enter Paradise." A Muslim must maintain ties with their kin and treat them kindly, even if they treat them badly or cut them off, for the one who maintains ties is not the one who merely reciprocates. Disputes over worldly matters do not justify severing ties of kinship. The right of parents over their child is immense, second only to the right of Allah Almighty, as He says: "And your Lord has decreed that you worship none but Him, and to parents, excellence." No matter how much parents fall short in their duty towards their child, the duty of righteousness and obedience, within what is good, remains incumbent upon the child.