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, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
It is permissible for that woman to keep her money with her brothers who, incidentally, might as well invest it for her during her life.
However, she must not ask them not to give the money to anyone after her death, because after her death the money will no longer be hers and will be owned by the heirs.
Also, she may not ask her brothers to withhold any portion of her money from her heirs, unless she makes a will transferring one third or less than one third of her wealth to a non-heir, such as the poor, the needy, or for charity. This is an act of charity that Allaah may guide her to observe so that her deeds increase after they terminate by her death. The Prophet, sallallaahu ‘alayhi wa sallam, said: “Allaah has given you authority over the disposal of one-third of your wealth at the time of your death, so that you may be able to add to the record of your good deeds.” [Al-Musnad and Sunan Ibn Maajah]
Allaah Knows best.