Answered by Mufti Mohammed Tosir Miah
Question:
My uncle died 2 months ago leaving no children. My aunty died before him. He has adopted his niece as his son and gave him his father’s name. Does this child have rights in his property? My uncle used to tell everyone that he will give one shop (out of five) to his adopted son’s son, who is handicapped. Is it possible?
Answer:
There are many virtues of looking after and taking an orphan child into one’s care. The Prophet of Allah 
said that the one who cares for an orphan would be with Him in Paradise, and he held up his blessed index and middle fingers with a slight gap to show how close together they would be. (Sahih Bukhari & Sahih Muslim)
Regarding your question, the adopted child would not inherit from your uncle as a son, but would inherit from him as a nephew if there is no one more closer to the deceased (uncle) than him. A more definite answer can be given if you were to provide me a list of the inheritors.
The reason why the nephew won’t inherit from the uncle as a son is because an adopted child is not considered to be one’s real son in the eyes of the Shariah.
Lastly, whether the nephew will be able to take a shop which the uncle promised depends on whether the uncle gifts the shop to him during his lifetime. When the uncle handed the gift to the nephew and he took it into his possession then this shop will be considered as the nephew’s and the other inheritors would not have a say in that shop. (Raddul Muhtar p.492 v.8)
If he did not give the shop as a gift during his lifetime, then the promise of a bequest would not be allowed as he is an inheritor and an inheritor is not allowed to be given a bequest as mentioned in a hadith which can be found in Sunan Tirmizi in where the Prophet of Allah said “there is no bequest in favour for an inheritor.”
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham