His siblings didn't know just how much Edmund had gone through at the hands of the Witch. Tumnus knew - he had also suffered. He had seen the worst that Jadis could do. Tumnus understood, but the others never really could. And Edmund vowed that they would not learn of it from him. Nothing anyone did could convince him to tell what had happened. So he was content in the knowledge that his siblings were blissfully ignorant.
He joked and laughed during the day. In the light, he could keep the darkness at bay. During the day, he could fill his mind with other things. Things like protecting the innocence of his siblings, especially his sisters, from the darkness they encountered during their reign - making sure that they never had to see the sorts of things he had witnessed, never had to learn the things that he learned. They would never know just what he was doing for them. He made sure of that. And it was better that way. They didn't know about the demons that taunted him from the shadows. He made sure that he always seemed cheerful.
But he was defenseless at night. There was nothing to drive the darkness away. He couldn't control his dreams. And in his dreams, his memories hounded him. One night he found himself shaken awake by Peter. His brother didn't say much. Perhaps he knew that Edmund would never explain. All he did was hold the Just King as he fought to regain control and calm down. Internally, he was panicking. Peter was never supposed to know about the nightmares.
When he went down for breakfast the next morning, Edmund knew that his brother had told their sisters. But he said nothing, and they asked no questions. They never did ask. But he knew that they knew. They didn't know the content of his memories and dreams, but they didn't have to. They knew enough. He didn't know, but they had suspected for some time. For no matter how he laughed and joked, he could not banish the haunted look in his eyes.