I agree, while Rahim Khan is shown to be a good person in the novel he is not completely innocent himself. While Baba and Rahim Khan both knew about the true relationship between Hassan and Amir, neither of the characters chose to tell the truth until they sought redemption for keeping the truth a secret for all of those years. However, Rahim Khan and Baba reacted to seek redemption in different ways which shows the development of their characters throughout the novel. Baba is shown to treat Amir very differently than the way he did in Afghanistan, possibly to make up for his poor treatment of him and never telling him about his relationship to Hassan. While in America, Baba becomes very proud of Amir as the societal and cultural differences allow him to prosper as a writer. Shown right before his death, Baba reads the stories Amir wrote kept in the leather book showing his full appreciation and love for his son; however, he never directly tells Amir that Hassan is his half brother. It is hinted that Hassan is of more importance than their servant when Amir attempts to get rid of him, when he asks Baba to get new servants Baba reacts very protective of Hassan and Ali and states that they are family (when in reality they really are family *Dun dun DAAAAAAAAHHHH*).
In Rahim Khan's case, he knew all along about the relation of Hassan and Amir and chose not to tell Amir despite how strong of a bond they share as he is the father figure for Amir in the novel. For his chance of redemption, he calls Amir and tells him to come visit him in Pakistan before his death. While he kind of uses Amir's own guilt to get him to travel to Pakistan, when he arrived he told him everything that had happened to make things right for both brothers. Khan tells Amir Hassan is his half brother and to make things right with Hassan, despite him being dead already, he tells Amir about Sohrab and how he needs to take him to a better orphanage.