بسم الله الرحمن الرحيم
You may adopt the son of your brother. In the question mentioned above the late Islamoddin has no child then his wife is eligible only for one fourth share in all the property left by him. It was wrong that the widow gave the house of husband to her sister as gift, since she has no right to do so. She may gift her sister or use herself as much share she owns in the house after she takes it into her complete possession, but it is not lawful to infringe in the share of other heirs. In the question mentioned above, the Shariah heirs of the deceased (parents, children, brother or sister) are not present and the adopted son is his bhatija then he has his right in the property. He may claim for his share.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.