[Sushi-Y]

Originally Posted by LostBlue

Shion forces Mion to switch names. Shion takes back her name as Mion (as she really born was Mion - at least if we were to trust her memory), thus forces Shion (formally Mion) to call her Onee as "Mion" is the superior one.

Now that you mention it, was this covered in the anime? I don't remember hearing it. Did they skip that part?

Spoiler for true identity of the twins:

Shion was actually born as Mion, and Mion was born as Shion.

Since "Mion" (that is, the Shion that we know) is the rightful heir to the Sonozaki family, she was favored over "Shion" (the Mion that we know) ever since they were little.

"Mion" and "Shion" disliked the different treatments they got: they're the same person, so why should one be treated different from the other?

That's why they began swapping identities so they could both enjoy the privileges that "Mion" has. Sometimes Mion would become Shion, and Shion would become Mion. In a way, both of them are Mion, and both of them are Shion.

However, one day, while "Shion" was pretending to be Mion, she was taken away and was given the demon tattoo which marked her as the next head of the Sonozaki family.

... Starting from that day, "Shion" lost her identity as Shion, and became Mion. "Mion" lost her identity as Mion, and became Shion.

That's why Mion has always felt bad towards Shion for stealing her identity, because it was Shion (the original Mion) who should've been chosen as the next head of the Sonozaki family, not her.

When Shion locked Mion up inside the underground jail cell, they finally reverted back to their original identities, hence the "onee" from Mion.

You can also see Shion referring to Mion as "Shion" in the final TIPs as well.

Now you know the true meanings behind Mion and Shion's coversation inside the library!