What should a person do who adopted a girl but did not inform her, and she is now fourteen years old? To what extent is he sinful, and what advice can be offered to him?

If the purpose of adoption is to attribute the girl's lineage to him, then it is haram and invalid, abrogated by Islamic law. This error must be rectified by attributing her to her father. If her father is unknown, she should be given a general name. However, if the purpose is to care for her and raise her without attributing her lineage to him, then it is permissible and rewarded. Nevertheless, the girl is considered a stranger to him after puberty, and she must observe hijab from him. What is forbidden to him from a strange woman is also forbidden from her, unless there is a reason for mahramiyah.