Is the reconciliation that took place after the intentional killing of the father, which the brother agreed to out of fear, legitimate and does it now prevent claiming the right, especially since some of the heirs were minors and were not consulted?

The reconciliation of a brother concerning his own right abrogates qisas (retribution) for the perpetrator, but it does not abrogate the right of the other guardians of the slain person to the blood money (diyya) unless they agree. So, whoever among them does not accept the reconciliation has the right to demand his share of the blood money. The partner then has a choice: either to forgive, in which case he receives nothing, or to join the reconciler, or not to join, in which case he is entitled to his share of the blood money for intentional killing, or to reconcile for a lesser or greater amount. The decisive authority in these matters is the Sharia courts or whatever acts in their stead.