She died and left behind a brother and two sisters
If this sister did not have any other heirs, and she did not have a husband or father or mother or grandfather, then her entire estate is to be divided among her living siblings, with the male getting a share equal to that of two females. So the brother gets half and each sister gets one quarter. Nothing goes to the deceased sister because one of the conditions of inheritance is that the heir should be alive after the person dies.
Allaah says concerning the inheritance of siblings (interpretation of the meaning):
“if there are brothers and sisters, the male will have twice the share of the female. (Thus) does Allaah make clear to you (His Law) lest you go astray. And Allaah is the All-Knower of everything”
And Allaah knows best.