ANSWER:

This is permissible and an act of kindness providing that the child keeps his father’s name and abides by the laws of sharia when it comes to issues of mahram and inheritance etc. 

QUESTION: Regarding adoption of a child i want to ask , if someone is willing to give there child to his brother/sister. What’s the ruling on this?? If this happened already what the other one should do now ? Should they give there adopted child back or keep it ??