Is it obligatory to inform the elder brother of a girl who is going astray, or does this matter not concern the questioner?

There is no harm in a man enjoining good upon an unmarriageable woman (ajnabiyyah) and forbidding her from evil. However, it is preferable that women or her mahrams (unmarriageable male relatives) handle this, for fear of corruption. Enjoining good and forbidding evil must be done with the hand; if unable, then with the tongue; and if unable, then with the heart, which is the weakest of faith. Forbidding evil may become an individual obligation if no one else is aware of it besides you, or if no one else can remove it but you. It is a condition that forbidding evil does not lead to a greater corruption than the evil itself.