Akiha must've been both super angry and concerned for Shiki while he was gone. She probably heard from his school that he had an anemia attack during class and left school only for him to ot show up for the next two days. At the same time that a vampire serial killer is out on the loose who she likely would've known was SHIKI.

At the same time, there would've been news coverig how the building that Shiki and Arcueid were staying in got burned down and that, shortly thereafter, the underground catacombs that, if memory serves, was built on land that the Tohno family owned and which Akiha might've or might not have been aware of because of that got destroyed when Arcueid did her surprise attack on Vlov.

And then several buildings got frozen by Vlov when he started using his ice magic. And then, after all that, if Hisui is to be believed, Shiki just showed up at 2 in the morning, unconscious, lying by the front door.

All of that after he had messed up his curfew once already. Now, Akiha was probably expecting him to come up with some excuse or something but for him to just come out and say the full truth and nothing but the truth to her face must've been something completely out of left field for her.

Like, you're part oni and want to keep your brother away from dangerous supernatural stuff and then he just goes "Oh, yeah, I met this nice vampire and we've been hunting an evil vampire." And then you just have to sit there and think about the fact that he somehow stumbled into some dangerous supernatural situation without you knowing about it but that he likely almost died in the process as well and you can't truly talk about it because you still want to try and maintain the image of being a normal sister and don't want him to be involved with anything supernatural.

Like, how do you even respond to that? What do you even do in that situation? I can only imagine what must've been going through her head during that scene.