Yeah, that’s a pretty shitty thing to do to your sister. Hiiragi owes Tsubaki a pretty massive apology. I could be mistaken, but it looks like Tsubaki passed on the elite school too (unless that’s the one that they’re at but seems not to be given Yokoe’s study habits). So she kept her sister from attending a better school, even though she was secretly taking an additional exam anyways, so why bother?
Yeah, we see on page 230 the prestigious school Tsubaki got into that Hiiragi apparently intentionally bombed was "Seirin Academy High School", whereas the main cast, including Tsubaki, all attends Nishitaga high school (incidentally, in chapter 79 we learned Kishiya-san attends Seirin Academy University -- and, on the bonus page, that it is selective [side note: this is a nice reference to A Kiss and a White Lily, where the main setting is the high school Seiran Academy, which also has an attached university]).
Call Hiiragi uncommunicative all you want and I'll agree, but setting Tsubaki up with the best prep school in the region was her way of showing that she actually cares about Tsubaki. She's clearly the type that hates talking about her own feelings and would rather hold a grudge for years than clear it up by talking about it. I'm sure you also know people like that. She also may be very smart but has no idea how other people work, pretty much the opposite of Kaname in that way.
I was originally thinking this was Hiiragi's idea -- get Tsubaki to go to a great high school without her -- and also her reason for the secrecy, but if that's the case, why wouldn't Hiiragi tell Tsubaki about the ruse before she turned down Seirin Academy? What was the point of going through the rigamarole of pretending to apply to a school so Tsubaki would as well if she was just going to let Tsubaki turn that school down when Tsubaki got in but Hiiragi didn't?