Is it obligatory to return the remaining amount of money allocated to the deceased son in prison to his two brothers and his father, knowing that he had no right to this money and it was allocated to him in anticipation of his release from prison, and his mother and sisters are alive?

The amount you allocated for your imprisoned son is considered an incomplete gift because he passed away before taking possession of it. Therefore, the amount remains your property. As for what you saved for him from the income of his two brothers working with you, it must be returned to them unless they give their permission. It is not permissible for a father to take anything from his son's wealth unless he is in need, and even those who permitted it stipulated that the father should not give what he takes from one son's wealth to another son.