twins are a complicated situation
they are the closest two people can be without being a single person, that breeds dependence
they are judged and compared to with a nearly exact copy of themselves, that breeds competition
without the careful navigation of the complex social/mental/behavioral/skill growth of a pair of twins, that dependence and competition will become resentment
in the case of the Hinase sisters, Tsubaki became insanely dependent on Hiiragi basically her shadow. and it is clear that Tsubaki stunted Hiiragis growth. Hiiragi definately felt the burdren of Tsubaki, but she was fine with that because she was "winning" the competition of acedemics, sports, and recongition... until she wasnt. And now the sister, who she sacrificed time/friends/opportunities to help, surpasses her, in everyway. She "will not lose", but she does. Now when they are compared Hiiragi is in Tsubakis shadow, she is losing the competition. To avoid comparisons Hiiragi stops doing things that Tsubaki does. Hiiragi cant be her own person because Tsubaki follows her whereever she goes even if it is actively hurting Tsubakis future and Hiiragis mental state.
This results in Hiiragi completely shutting down, she quits her hobbies, her grades slip, and she isolates herself. Her choosing a different school is for her to be her own person, unlike Tsubaki, Hiiragi doesnt like the idea of just being "Tsubakis younger sister"
Hiiragis asshole attitude towards Tubaki is totally understandable and the people saying this backstory makes her look worse lack empathy she is obviously been depressed and angry for a long time. Hiiragi is a very strange character to have in a primarily gag manga, because her character is actually very sad and not much fun can be had with a genuinely depressed character.
Hiiragi is more fit as a depressed heroine that Ren needs to "save" in a romance story
