Is the woman considered religiously sinful for adopting an illegitimate girl and concealing her from her real mother? Is her brother justified in demanding that the mother be allowed access to her daughter? And what should she do?

Sponsoring the child and being good to her is a good thing, but adoption (تبني) is forbidden. Allah has made the care and custody of children obligatory, and the mother must bear her responsibility in that. It is not permissible to prevent the mother from seeing her daughter; rather, her desire to see the girl and monitor her condition must be fulfilled. If she sees her neglected, then custody returns to her.