All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
It is more appropriate for the disputing parties to reconcile on a solution that would please all of them in regard to this inheritance in order to block the door to hatred and dissension. Reconciliation is permissible and it is good and more appropriate, especially that resorting to the division of inheritance according to the Sharee’ah may put you in great embarrassment as it has been a long time since the grandfather had passed away, as well as the father who was managing this wealth, and it is difficult to find enough evidence to prove the rights of the heirs.
However, if you insist on the division of the inheritance, then you should take your case to an Islamic court, as a mere Fatwa issued according to a question sent from a distance is not enough.
In any case, in general the amount of the inheritance should be exactly known as it was on the day the grandfather died, and then the inheritance should be divided between all the heirs according to the Sharee’ah, each according to his/her due share. So, the share of each and every heir should be known.
Therefore, the wealth (inheritance) should all be gathered and known well; then what is a surplus to the share of your father should be taken and given back to his sisters. So, what is still existing should be given to whoever is eligible for it, and whatever has been consumed by selling it or otherwise, then it is a debt and a liability that should be paid and guaranteed according to its value on the day of the division of inheritance, because what the father really did was that he was a partner who disposed of the share of others without their consent, so he is like someone taking something illegally by force.
As regards the benefits, like the profit of real estate from living, renting and other items of inheritance: then your father should have guaranteed all of it according to the market price of real estate similar to it, so this should be determined if possible, otherwise its value should be estimated and then divided according to the above.
On the other hand, if the money of the inheritance was used in business, then if there is a loss, he (your father) should have guaranteed whatever was lost from it, and if he gained any profit from his business, then the scholars differed in opinion in this regard. Some said that the profit (i.e. the increment that resulted from that money) is for whoever has taken the money by force in return for guaranteeing the capital money for whoever he has taken the money from wrongfully, and some said that the profit is for the person from whom the money was taken as it is a profit that resulted from his money, while some other scholars – like Ibn Taymiyyah – held as preponderant the opinion that the profit should be divided equally between the owner of the capital money and the person who has taken the money from him wrongfully. For more benefit, please refer to Fatwa 156229.
Allaah Knows best.