Complex sentence: In 1909, Köselitz left the Archiv and never again spoke publicly about it; in private letters, he condemned Nietzsche's sister, as he had done prior to his working for her.
Split into simpler sentences: In 1909, Köselitz left the Archiv and never again spoke publicly about it. In private letters, he condemned Nietzsche's sister, as he had done prior to his working for her.