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.
You did not clarify to us whether the brothers and the sister are all full-brothers and a full-sister, or are all from the father’s side only, or are all from the mother’s side only, or that they are mixed, because the inheritance differs according to the difference of their circumstances. If we presume that they are all full-brothers and a full sister or that they are all from the father’s side only, then a woman who died and left behind three brothers and a sister –and she did not leave any other heir –like a father, a grandfather, a mother or grandmother –then her inheritance will be divided among her brothers and sister, the male twice the share of the female, as Allaah Says (what means): {…if there are brothers and sisters, the male will have twice the share of the female. Allaah makes clear to you [His Law], lest you go astray. And Allaah is Knowing of all things.} [Quran 4:176]
The share of the dead sister in the house and all the gold, silver and cash, and other items that she possessed will be divided into 7 shares; each brother gets two shares and the sister gets one share.
The claim of the sister that the property of her dead sister becomes her property needs more explanation. Did she claim that she had bought them from her or that the dead sister had gifted them to her, and whether she has evidence about her claims?
Therefore, you should take this claim to an Islamic court, if any, or present the case orally to the scholars.
Allaah Knows best.