In the question mentioned above, if at the time of your father’s death none of his parents was alive then all the property of your father shall be divided into 64 shares after paying due rights proceeding inheritance. Out of which your mother shall get 8 shares, 14-14 shares shall go to each son while each sister shall get 7-7 shares. The detail of Shariah division is as follows:
It should be noted that the monthly pension that your mother is receiving is solely her right, it is a kind of help and gift from the government for her. Neither it shall be considered as the inheritance of the deceased father nor shall it be distributed among his Shariah heirs.
Allah knows Best!
Darul Uloom Deoband