How do I keep a secret between my characters, so the AI does not reveal it too early?

From version 1.9.22, open the Codex from the book's page, choose Relations and scroll down to "Secrets: who knows what". Press "Add a secret" and write it, for example "Tom is John's son". Click the people who know it from the start. If someone finds out later, press "Someone learns it", choose the person and the chapter where they learn it: until that chapter, the AI never lets them act on it. "The reader is told in chapter" keeps the narration to hints before that chapter. Press Save. The consistency check also flags a chapter where someone seems to know a secret before they should.