Is a wife forbidden to her husband, like the prohibition of a sister, if the husband made that contingent upon a certain action, and then the action occurred without the wife's knowledge?

If you intended to forbid your wife to yourself, but did not utter it, then this intention has no effect. However, if you uttered it when you were alone and your wife did not hear you, then this is a conditional dhahir which takes effect when the condition it was tied to occurs. You must offer the expiation for dhahir before having intercourse with her. And you must repent to Allah for this forbidden utterance.