Is it permissible to inform a brother about the presence of his sister's pictures on Twitter?

You must verify that your relative is indeed publishing the photos, as it might be unintentional or done by someone else, and manipulating programs and hacking them is possible. If the act is repeated intentionally, and the photos display adornment and invite temptation, then this is a reprehensible act (munkar). She should be advised in a kind manner and reminded of the seriousness of the act, the necessity of repentance, and the attempt to remove the photos. If she does not desist, she can be warned about the exploitation of these photos for blackmail and defamation. If she still does not respond, her guardian should be informed to take the necessary measures to stop her.