So his sister is the fortune teller they met at that festival. They'll interact with her more later in the story.
As far as reading the novel, I wound up finding a spoiler thread on Novel updates (don't think I can link it here), where a user posted translations/summaries of the novel from vol 2-5. There's a total of 5 volumes if I remember correctly. Apparently, the last group that was translating it got C&D and no group has tried to translate since. The thread also had instructions for buying the novel from Ridibooks, so if you don't mind MTL yourself, that's also an option. I just bought the book to support even though I can't read it lol. [\spoiler]