After a while, she went up to her future father-in-law and asked him, point-blank if he was a Mafia boss. He denied it categorically. But she was not convinced — and broke up with her fiancé. Within a couple of hours, Don Vito appeared before her, saying: “You can do whatever you like, but sooner or later, you’ll be my daughter-in-law.”

“Basically, this man was threatening me, saying that if I didn’t marry his son, he would kill my family. In those days, this was all perfectly normal,” Ms. Aiello said. By now 18, the young woman decided to marry without telling her family, in the hope that things would improve. Yet eight days after the wedding, her father-in-law, Don Vito, was murdered.

“That’s when the apocalypse began, because my husband swore vengeance,” she said. He started going around criminal circles to track down his father’s killers. Four years after Don Vito’s murder, Ms. Aiello’s husband found the killers. Ms. Aiello was 22 and the couple had a young daughter.

But his adversaries got him first. “They shot and killed him in front of my eyes, in a pizzeria that we managed,” she said. She recognized the killers clearly: There were two inside the pizzeria, and two accomplices outside. In the four years that she had spent living with her husband, she had had to witness plenty of other criminal activities that she disapproved of. So she decided to become a witness against the Mafia and turn in all the guilty parties.

Her chief mentor in this enterprise was Paolo Borsellino, a judge and prosecuting magistrate based in Palermo who dedicated his life to fighting the Mafia, and who was assassinated by a car bomb in Sicily in July 1992.