بسم الله الرحمن الرحيم
According to Shariah, an adopted child is neither an heir nor have any right in the property of that person who has adopted. Moreover, according to Shariah, it is disliked to deprive one’s heirs completely and give all the property to adopted child. Hence in the question mentioned above your big brother should not give all the property to adopted child (whether he intends to give both the daughters or the son as well), if he intends to give then he should give him small quantity as per need and keep the remaining for other heirs which will be divided among his legal heirs (i.e. brother, sister, wife etc) as per the Shariah rules.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.