What is the solution when a sister who is immodestly dressed speaks with a young man via phone and the internet, and the brother fears disgrace, while the parents do not want him to intervene, and advice has proven fruitless?

The brother must advise his sister and forbid her from evil, and warn her of Allah's punishment. He must be patient and choose appropriate methods for advice. If she does not respond, he should gently advise his parents and remind them of their duty and responsibility towards their daughter.