I think what really fucks with the narrative is that it starts out with normal conversational bubbles in the "present day" on white background, then goes into square narrative boxes with a flashback on black background for how they started to go out and her thoughts on society, then it ostensibly reverts to the present on white BUT it keeps the narrative boxes style and summarizes meeting her mother and how things unfolded from there. So it feels like the "present day" only exists at the very start of this oneshot.
Clearly 11 pages wasn't enough, they should have had another 5 pages to properly lay out the present day's status quo so they wouldn't need to summarize events like that. Though in the end, all these flaws forced me to think more about the manga, so you could say I got more out of it.
As for what's going on... Had to reread it, since every line is hinting at everyone's weird complexes.
The introvert main girl not caring who she dates as long as nobody else exists to pressure her about it, clearly feels that society is oppressive; maybe not even gay. Doesn't feel loved by parents. Just wants to be loved/needed by anyone, so she agrees to go out with other girl and agrees to whatever she wants.
The other girl is insecure and wants to dominate someone else to compensate, I guess? It's not laid out completely clearly. Maybe she thought that if she dominated her "sister" then she wouldn't have to feel inferior to her dead sister. Nice feet, though.
Mother is just... yeah, she has the same vacant eyes as that one dude's mom from Yofukashi no Uta. No explanation needed. It makes foot girl Kana feel terrible and vulnerable, obviously. Though main girl doesn't give a shit, she's just smiling because she is thinking to herself now that maybe she truly "loves" Kana because Kana is able to make her "irreplaceable" presumably by connecting her up to a whole family that will be emotionally dependent on her. Feels like yet another manga dunking on lesbian relationships.
Final scores:
Kana = actually not that bad all things considered, just a lonely tsundere
Mom = deranged personality, but very functional
Unnamed main girl = possibly the craziest, if she actually is literally thinking everything in the narrative boxes; too self-aware...