Big sister is scary, but at least doesn't seem to be an asshole. Maybe just overly protective.
I think overprotective's exactly right. She seems to be
stalking watching over her in the cover art. She probably won't approve of
any of Shiki's friends, because no on can meet her standards of acceptable friend.
Koto's
really cute in that final image, long hair looks good on her. It probably helps that she looks happy for a change. It's interesting that Kaede looks so much younger than the others in that one, even though she's only a year younger. Without the uniform, she'd look like an elementary school girl. It's interesting that of the high school girls only Shiki is wearing her skirt short. All the rest are wearing it properly.