There are 2 real brothers X and Y. Y had adopted the daughter of X during the life of their father through mutual consent of both these brothers and their father. It is now the time to marry this adopted girl and the two brothers developed family feud and are totally opposed to each other to the point of animosity. As an authorised representative (waqeel) of Ayatullah Sistani, please advise as to whose permission is now necessary to get the girl married – X (the real father) or Y (her uncle and adoptive father)? Can X claim her daughter back at this point and take care of her marriage alone?

Adoption is not accepted in Islam, and it is void (The adoption which is permissible is not real adoption, it is only a custody.)
The permission has to be taken from the father (X), and he can take his daughter, as his brother has no rights at all, even if the father died, in which case, the daughter can marry her self without referral to the uncle.