Kasane's role in this kinda feels like - a person who's super into yuri manga got Isekai'd into a yuri rom-drama between a siscon younger sister and her unrequited love for her onee-chan, and Kasane is the MC who unwittingly got inserted into their lives as the "replacement", but has not actual attachment to anyone and just wants all the yuri ships to work out somehow - only the younger sister fell for her in the process while the MC remains blissfully unaware of this fact.
I know she also likes Meguru to some degree, but this chapter kinda feels like a very strong argument in favor of the "Kasane would rather ship yuri than experience yuri", if she's brainstorming how to destroy a marriage an ocean away for the sake of one high school girl's happiness.