Dang, what with the hate on Hiiragi ? What she does is pretty understandable. Imagine having your twin who always relied on you suddendly becoming better than you at everything but still thinking she is worthless without you. Hiiragi is just angry that her sister can't realize how good she is and instead insist on puttting herself down. Imagine how frustrating that is, having someone who is better than you but insisting on telling she is bad, it's extremlyfrustrating.
If i agree that not telling Tsubaki she didn't want to go in the same highschool was not the best thing, you have to understand that Hiiragi know that Tsubaki would have follow her anywhere and continuing putting her in her shadow.
It's for the best like Tsubaki mention, if it mean Hiiragi can be happy then maybe it's better if they're in differents highschools.Plus they see each other at home anyway.
This situation remind me of the Hino/Saya relation is often portrayed in more serious Bang Dream doujins.