Complex sentence: Jamie points out to Edmund that they had concealed their mother's addiction from him for ten years, when he was a child, and so his naiveté about the nature of the disease was understandable but deluded.
Split into simpler sentences: Jamie points out to Edmund that they had concealed their mother's addiction from him for ten years. Jamie explains to Edmund about his naiveté about the nature of the disease was understandable but deluded.