[SpiritofTheNaiker]

Originally Posted by Sir Maddy

Spoiler for Kind of an answer - hope it helps (sorta):

Well considering how delusional Shion is at this point.
I'm not surprised if she would of thought that she was Mion.
And if she did show them her back, pretty sure that would conclude she was Shion.

Spoiler for My hypothesis about it:

Could be possible, instead, that Watanagashi-hen and Meakashi-hen are not the same world from multiple points of view, but just different worlds, with nearly the same plot but different villains?
Namely, in Watanagashi-hen, could the guilty be really Mion, and not her sister? That scene would get a sense, with this theory, she wanted to show her back because there was really a tattoo on it. (while in Meakashi-hen there is not such a scene, as a matter of fact). After all, in one of the first episodes of Kai, they had won in the "Zombie tag" game, saying the had thought about the same strategy, due to the fact they were twins.