I think people are really misinterpreting the tone of this chapter. The feeling I get from the last panel isn’t her going yandere mode or something, and she’s not being a petulant brat either. She’s upset because she’s tried really hard to be kind and supportive to the person she cares about the most and has never had those feelings reciprocated. And now that person has finally been able to open up like Tsubaki always wanted her to, but only for someone else. So she’s gotten what she wanted—to help her sister be happy—but she wasn’t able to be the one Hiiragi connected with despite trying for years. It’s a bittersweet melancholic feeling, and she probably feels somewhat guilty as well for being selfish. It’s genuinely a really well-written scenario.