Astrid Holleeder agreed to speak with The New York Times only from a separate, secure location, and she said that she did not think she would ever be safe as long as her brother was alive.

“He will not rest until we’re dead,” she said. “It’s about his pride. He cannot let his little sister take him down.”

Ms. Holleeder said that she wrote “Judas” as a kind of “last will and testament” for her daughter, who is 31 and has two children of her own. She said that testifying in court during the pretrial phase of the current case had so far felt ineffectual. (The trial is scheduled to begin in early 2017.)

“I want to tell everything, but they haven’t given me the chance yet,” she said. “The book is just about how I feel, how we grew up, what in fact made me and my brother into the opponents that we are now.”

Even when the 570-page manuscript was finished, Lebowski Publishers did not provide details of its contents to bookstores, for fear that Mr. Holleeder would try to prevent its release. But after the publisher disclosed information on a late-night television talk show the day before the book’s publication, the first print run of 80,000 copies sold out immediately. About 400,000 copies have been bought so far — a milestone in a country of about 17 million, where sales of 5,000 copies are considered strong.

“Judas” is a narrative in the present, with flashbacks to the Holleeders’ childhood. The account begins in January 2012, when Mr. Holleeder was released from prison after serving six years of a nine-year sentence for extortion. Astrid Holleeder said she acted as a kind of consigliere for her brother — advising him on legal matters, arranging his safe house when he got out of prison and serving as a confidante — in an effort to get close enough to obtain material that she could share with the police. She started wearing a wire in 2013, and ultimately recorded hundreds of hours of conversations with her brother.