Under provisions of Hindu adoption and maintenance act
No person can be adopted unless
He or she is a Hindu;
He or she has not already been adopted;
He or she has not been married, unless there is a custom or usage applicable to the parties which permits persons who are married being taken in adoption;
He or she has not completed the age of fifteen years unless there is a custom or usage applicable to the parties, which permits persons who have completed the age of fifteen years being taken in adoption
2) in your case your brother could not have adopted Muslim man
3) adoption is not valid