While fake spirit mediums are common (basically just petty scams), one with any credibility ought to be much rarer. So, it's probably not as unlikely it just happened to be the little sister as Shiraishi might think.
I really enjoy this one, the themes are pretty well connected and balanced, it has fun without losing touch with the central conflict. Yes, this chapter is silly, but she's still dead and hanging around. There's a lot going on "below the surface" in this one.