I'm about 20% of the way through "A Clash of Kings" and I'm confused about Lord Stannis' right to claim to the throne.
Throughout Game of Thrones, we learn that Jon Arryn learned of the true father of Cersei's children from the book he was reading and from seeking two of Roberts' bastards. We similarly learn that Ned came to the same conclusions from the same sources.
We are also told that Ned tries to send a letter to Stannis with his findings, but that is intercepted by the queen.
But in "A Clash of Kings", we are told Stannis somehow knows the truth, yet he had only met one son of Robert. How does he know what nobody else, including his brother, seem to suspect? And why has he never raised his suspicions with his brother?