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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
In principle the wealth that a deceased leaves is for his/her heirs after his death. The Prophet said: "Whoever leaves any wealth is to be divided among his/her heirs." So it is an obligation to enable her heirs to divide her wealth among them according to the Islamic Law after paying any debts that she owed and fulfil her will if she had left any. The condition of a will is that it should be limited to one-third of the wealth and it should not be dedicated to an eligible heir. If your sister had offered some of this money or all of it to you as a gift, and she was in her full senses when it was given to you, and you took full control of it when she was still in her full senses, then it is permissible for you to benefit from whatever she had given you. You have to know that the most important thing that is going to benefit your sister is to supplicate for her, and if you put some of your money in a continuous charitable field then that is good.
Allaah knows best.