I actually find that theory plausible because I think there are currently two Win-nas right now.
In chapter five, in the same night, it's evident that
Tsundere Wi-na is at home with Wi-jin,
And
Yandere Wi-na is hiring the fat delusioner to kill Shi-an.
*
As well as in chapter nine, where
Yandere Wi-na is wearing the school uniform on a rooftop:
Then at the hospital,
Tsundere Wi-na is wearing entirely different clothes:
And
Tsundere Wi-na was waiting at home in chapter eight:
*
So maybe Yandere Wi-na is the real one and she created a delusion that lives with Wi-jin or maybe she creates a delusion whenever she does something behind the scenes and just erases Tsundere Wi-na away when she comes back home.
*
Even in chapter three where Wi-jin made a delusion of Wi-na, there was a side to that delusion which wasn't apparent in
Tsundere Wi-na that lives with him:
*
I interpret that Wi-jin accidentally made a delusion of Wi-na that is actually the real character of his sister at the school infirmary. This could mean that*
Yandere Wi-na is the real one, and she could have made a delusion of herself (which is
Tsundere Wi-na) that lives with Wi-jin that has an entirely different persona (Tsundere-like) to remove any suspicious behaviour.
*
In chapter five,
Yandere Wi-na is probably the delusioner they're talking about that is causing the accidents because the scene is followed by
Yandere Wi-na ambushing the ambulance:
So maybe she uses
Tsundere Wi-na as an alibi whenever she causes these accidents.
*
Yandere Wi-na also says "Join us."
Maybe she's implying that she's part of a group that are against Wakers or something.
*
The opposite could also be plausible that Tsundere Wi-na is the real sister and Yandere Wi-na is his delusion that could have become real. And so,
Yandere Wi-na somehow became obsessed with her brother and is now trying to destroy anyone (Wakers) that may prevent her from being with him.
*
Even the delusions of Wi-na that Wi-jin makes are always in a school uniform, like in chapter one and three:
These delusion-made Wi-nas are shown to be clingy and crazy as well.
And
Yandere Wi-na is mostly shown to be in a school uniform like the night she was recruiting the fat delusioner and when she was on the rooftop.
*
Although, in chapter 1.5, Wi-Jin created a delusion on the school rooftop but she was... well... naked.
*
Nevertheless, I conclude that
Yandere Wi-na is his "ideal" version of Wi-na, and this Wi-na unexpectedly became crazy in love with her Oppa.
*
It's evident from chapter one that Wi-jin is such a powerful delusioner (both a plus and a negative delusioner - Ch.12) that even sometimes his delusions spawn without him being concious of it. Maybe
Yandere Wi-na is simply a delusion that is now way out of his control.
*
Or maybe
there are actually multiple Wi-nas that Wi-jin has unconsciously created. The author is probably hinting that there are more Wi-nas just from chapter one:
And chapter 1.5
*
Either one of them is the real one, or none of them are real at all.