There's definitely been some serious falling out between the twins that we haven't yet been told of (possibly what resulted in Yatsude encountering the crying Tsubaki in which case it'd be quite recent and the wound still raw and bleeding) because Hiiragi's undiluted rancor is way above and beyond normal sibling squabbling, even allowing for their majorly conflicting personalities.
Tsubaki's huge siscon streak may well be a contributing factor if Hiiragi knows of it (which seems likely)...
Sounds like the moody one just doesn't like to be with anyone.
Unlike Tsubaki, who's mostly friendless due to severe social anxiety issues and visibly suffers from it, Hiiragi gives the impression of being solitary out of preference and mostly plain uninterested in socialising with people - aside from Sensei who she ironically pesters not unlike Utsugi does her. Though she does seem to have started lowkey warming up to Utsugi's persistence rather despite herself, which she seems to find perplexing.