This is probably not Julia Craddock Brumfield's brother John David Craddock (b. 1858), since it is unlikely Julia would have referred to "John Craddock's sister" if John Craddock had been her brother.
The graph displays the other subjects mentioned on the same pages as the subject “John Craddock”. If the same subject occurs on a page with “John Craddock” more than once, it appears closer to “John Craddock” on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.