I'm wondering how Tsubaki has gone from mind broken to a total siscon. I mean Yokoe didn't change that much after geeting érejected" because she was already a bit of a goofball before but Tsubaki wasn't that much of a siscon before Hiiragi choose another school.
Her whole life she thought she had a good relationship with her sister, her entire life then crumbled when she startet to think that Hiragi might have hated her All along (pretty much their entire life), afterwards Hiragi got even colder towards her as seen in their non flashback interactions, its not hard to see why she would become desperate to regain that relationship she thought she had