My Sister and I is an apocryphal work attributed to the German philosopher Friedrich Nietzsche. Following the Nietzsche scholar Walter Kaufmann, most consider the work to be a literary forgery,although a small minority argues for the book's authenticity.
It was supposedly written in 1889 or early 1890 during Nietzsche's stay in a mental asylum in the Thuringian city of Jena. If legitimate, My Sister and I would be Nietzsche's second autobiographical and final overall work, chronologically following his Wahnbriefe (Madness Letters), written during his extended time of mental collapse. My Sister and I makes several bold and otherwise unreported biographical claims, most notably of an incestuous relationship between Nietzsche and his sister Elisabeth Förster-Nietzsche, as well as an affair with Richard Wagner's wife Cosima. It is written in a style that combines anecdote and aphorism in a manner similar to other Nietzsche works.
The book was tied quickly to controversial publisher Samuel Roth, the putative owner of Seven Sirens, who had spent jail time for the unlawful distribution of a version of James Joyce's Ulysses (1922). In the book's introduction, an anonymous publisher claimed to have received the manuscript from a fellow inmate of Nietzsche's in Jena and to have hired Levy to translate the work only to have both German and English manuscripts confiscated, with only the latter surviving.

In a response letter, Levy's daughter vehemently denied her father's involvement with My Sister and I. Kaufmann claimed in a footnote in his Nietzsche: Philosopher, Psychologist, Antichrist (first published 1950) to have received a ghostwriting confession from minor author David George Plotkin in 1965.

Reception

Nietzsche scholars in general adopted the opinion of Kaufmann, who immediately identified the book as a forgery in a 1952 article. Evidence against the book cited both by Kaufmann and later commentators includes anachronisms, such as a reference to an 1898 incident, incongruous references to Marxism, and the city of Detroit (globally unknown in the late 19th century), along with a seemingly poor grasp of philosophy and sexualized pulpy content. Kaufmann also notes instances of clever wordplay in this English text that are impossible to express as such in the German language.

Since the only copy of this alleged work is in English, and there is not a single page of what would have been his original in German, opinion has been largely that the work is a forgery.
https://en.wikipedia.org/wiki/My_Sister_and_I_(Nietzsche)