Hawzah News Agency - What is the stance of Islam in regards to adopting children and if a child is with the family from his or her early days and develops a bond, do the laws of Mahram and Non-Mahram still have to be observed?
Answer: Adoption in the sense of claiming a child as your own son or daughter and getting official legal documents for him or her in your name is not permissible.
However, there is no Ishkaal (objection) in taking care of a child, raising him or her and showing love and care, which is in itself rewarding but the child's lineage has to be preserved.
In addition, the child would remain non-Mahram once reached the age of Buloogh (puberty).
At the appropriate moment, the child should be informed about it (being adopted0 and if one cannot inform the child, he should record it somewhere or ask someone to bear witness that he/she is not his child.
sistani.org