They are the monsters that each other made...
That being said, I have to agree with the others here; this backstory makes Hiiragi sound much worse than before. Reading between the lines, she was kind and accomodating to her sister only as long as her sister was beneath her; the lesser, always racing to catch up. As soon as Tsubaki proved her equal, that kindness turned to coldness, then resentment, then sabotage. And Tsubaki, blind and kind as she is, doesn't understand that, can only imagine that this is all her fault somehow, that she has committed some terrible crime she still needs to atone for.
In previous chapters, it felt like Hiiragi's inferiority complex was externally motivated. A slowly festering wound, built over years. But here, we see that wasn't the case; her aggression was damn near instant. As soon as she came second at a single thing, she snapped at her sister. If she blames other people for that, she's projecting - the fault comes from within.