What should a brother do with his adult sister who is speaking to young men in an immodest way and going out with them?

The brother must advise his sister and explain to her the prohibition of her actions, urging her to repent and become righteous. If she repents, he should conceal her (faults). But if she returns to reprehensible acts, he should inform her father, if he is present and if he does not fear a greater evil, so that her father may deter her. If that is not possible, he should exert his utmost effort to prevent her from reprehensible acts as much as he can, and he is not permitted to remain silent. This is based on the hadith, "Whoever among you sees an evil, let him change it with his hand...", and because changing evil may become obligatory upon whoever becomes aware of it. A Muslim must teach his family matters of their religion, enjoin them to good, and forbid them from evil.