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 is His slave and Messenger.
If your mother did not leave an heir other than yourselves, like leaving one of her parents alive for example, then the inheritance of your father will be divided amongst you, the male twice the share of the female. Therefore, the share of the son will be two shares from twelve shares, and the share of a daughter is one share from twelve shares from the inheritance of your father, and there is nothing for your mother, because you inherit her according to the same division.
However, if your mother had left one of her parents alive, then he/she [parent] gets one-sixth of her share from her inheritance from your father, which is one-eight, and the remainder of her share will be divided among you according to the previous method.
Allaah Knows best.