Before an audio tape of President Javier Milei’s sister was recently leaked to the news media, most Argentines had never heard her voice.

Karina Milei, perhaps the second most powerful person in Argentina, had largely kept silent, pulling strings from behind the scenes while her boisterous brother commanded the stage.

But now Argentina’s mysterious first sister, who quietly helped fuel her brother’s rise from TV pundit to president and a leader of the world’s populist right, has become a focal point for a public test of his government and a lightning rod for corruption accusations leveled against it.

While the tape attributed to her was innocuous, rallying party members and telling them to stay united, a separate tape attributed to a different government official suggested she was profiting from a bribery scheme.

And all of this has emerged just as she was organizing Milei’s party’s campaign for a crucial midterm vote this fall — a key test of her brother’s transformative plans for the country.

But just as he has done since they were children, Mr. Milei, 54, has stood by his sister, denying the accusations, and there are no signs that her power or influence have weakened.