Is what the claimant took from the inheritance a legitimate right for him, and has he thereby taken what is not rightfully his, especially given the cooperation of all siblings in developing properties and accumulating wealth before and after the father's death?

If everything has been divided and the family members, who are competent to dispose of their property, have reached a mutual agreement, then the division is legally valid and binding, even if it involves some unfairness. What you have obtained from this division is a legitimate right. We advise you to fear Allah, maintain kinship ties, and reconcile disputes, as Allah Almighty says: "So fear Allah and amend that which is between you and obey Allah and His Messenger" (Al-Anfal: 1), and as the Prophet (peace be upon him) said: "Reconciling disputes is better than the status of fasting, prayer, and charity."