Is the questioner or his heirs entitled to claim the amount that was paid to the younger brother under duress after the father's death, and how can the value of these funds be estimated at that time?

It is not permissible for a father to take his son's money to give it to another child or to force them to do so. The fundamental principle is the impermissibility of taking another's money except with their willing consent, based on the Almighty's saying: ﴿O YOU WHO HAVE BELIEVED, DO NOT CONSUME ONE ANOTHER’S PROPERTIES UNLAWFULLY EXCEPT THAT IT BE TRADING [CONDUCTED] BY MUTUAL CONSENT AMONG YOU﴾, and the Prophet's (peace be upon him) saying: "The property of a Muslim is not lawful unless given with his willing consent." Whatever was taken from your money without your consent is considered unlawful usurpation, and it is not permissible for the brother to take it or to demand what equals the interest of a usurious bank. Rather, he is only entitled to his share of the profits. This money remains a debt owed by the brother. If he invested it and made a profit, you are entitled to a share of the profit (on the basis of Mudarabah – profit-sharing partnership). If he incurs a loss, he is still liable for the principal amount. If the currency depreciates by a third or more, he is obligated to compensate for the difference. The advice is to uphold ties of kinship.