It kinda feels like Kasane is just an otaku who got Isekai'd into a yuri manga story of a siscon high schooler mourning her unrequited love for her onee-chan; and Kasane got herself unwittingly mixed up with Meguru and found herself becoming the little sister's "stand-in onee-chan". But then Meguru found herself starting to fall for Kasane instead, but our Isekai MC is too busy shipping the little sister with everyone else, and is thus oblivious to the reality of her situation.
And she likes Meguru to a degree, but is so focused on the absract ideal that is yuri, that Kasane fails to see the plot happening to her, and instead thinks of it as happening around her.
I know that's not actually the case, but this chapter especially makes a pretty compelling argument for it, when she spends her whole workday brainstorming how to break up a marriage half the world away, just to try and make a siscon high schooler happy.
Edit: - also I don't know how I looked at "Kanase", thought 'this sounds wrong but I don't want to go look up how to actually spell her name right now', and proceeded to type it out four times before going '....ah fuck me' and having to go back and fix it.