بسم الله الرحمن الرحيم
(Fatwa: 400/412/N=04/1435)
According to Shariah an adopted son is not the Shariah heir of the person who has adopted him. Those who consider it heir they are wrong. In the above mentioned question, the heirs of each of the sister are their alive sister and bhatijas. It is usurpation and infringement of rights to give all the property to a person who is not an heir. However, if they give something from their property into your complete possession in their lifetime then you shall become its owner and it is lawful as per the Shariah.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband