Uhm... almost total agreement here, just that I thought they had a little sister/big brother chemistry more than a niece/uncle chemistry.

Well, anything BUT romantic chemistry. To everybody who says this can evolve into more, I only say: I like my incest nice and GAY, thank you very much. :)

Also, I really want to know how they always pick the one actress that has the least sexual chemistry with Jensen Ackles to play his (potential) love interest. Because I don't get it.

I have asked myself this very question because holy hell, I don`t think much about the acting talents of JAlba and KK but at least Jensen had chemistry with them. As did he with a nice percentage of the weekly chicks.
Why this perverse glee in making the love interest chem-free?

And I really liked the horror atmosphere of the episode.

Hm, I was really psyched about reading the H.H.Holmes story, I had imagined more stuff in an actual horror mansion. And since I didn`t care about Jo, her Kill Bill deal left me cold.
As it was the most atmosphere I got was from having low-quality, blurry clips that at least looked like an old horror movie.