Why does Hiram know that R.C. is innocent?

RC told Hiram that he was innocent. On the way to get fishing gear in the shed, Hiram runs into one of the Remington brothers. After a circular conversation, Hiram finds out that the other brother spotted Grampa with the accused murderers on the night Emmett went missing. RC wasn't there. Once Hiram goes back in the house, he confronts Grampa with this information, which Grampa admits but is unapologetic for being a part of Emmett’s kidnapping.