In To Kill a Mockingbird, who dies in Chapter 8 and who "replaces" him?
Your question confuses two separate deaths in the Radley family. In the beginning of Chapter 8, we learn that Mrs. Radley died that winter, the winter when it snowed in Maycomb and when Miss Maudie's house burned. It is in Chapter 1 that old Mr. Radley died at home. The children, along with the rest of the neighborhood, thought that Boo Radley might come out in public since his father had died, but that was not to be. Instead, Nathan Radley, Boo's elder brother, moved from Pensacola to Maycomb to run the Radley house and supervise Boo.
According to Scout, there was not much difference between the two, which suggests that Boo's life inside that house did not change. Nathan Radley would speak to Jem and Scout, unlike his father, but he showed himself to be very controlling and cruel in his treatment of Boo. This side of his character was made clear when he cemented the hole in the tree for no other reason than to keep Boo from communicating with the children. When old Mr. Radley died, Calpurnia said he was "the meanest man ever God blew breath into," a strong statement from Cal who never offered her observations about white people. Based on the incident with the tree, Nathan Radley appeared to be very much like his father before him.