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.
Inheritance is only confirmed by the death of the person whom one is entitled to inherit and the heir who died before taking his share from the inheritance or after taking only a portion of his it share, then the remainder of his share will be transferred to his heirs as his death does not mean that his due share is dropped. Therefore, the share of your mother is transferred to her heirs, and the share of your two sisters is transferred to their heirs and the same thing applies to the share of your brother; there is no difference of opinion about this among the scholars praise be to Allaah.
Finally, it should be noted that the matter of inheritance is very complex an issue, so a mere Fatwa which is an answer issued according to the question is not enough. Rather, the matter should be taken to an Islamic court to look into the case, or it should be presented orally (in person) to the scholars in case there are no Islamic courts. It might be that there is an heir who would not be known except after investigation, and there might also be a will, debts, or other dues that are not known to the heirs. It is known that these rights come in priority over the right of the heirs. Therefore, the inheritance should not be divided without resorting to an Islamic court, if any, in order to fulfill the interests of the dead and the live people.
Allaah Knows best.