>>279764126
If you take it on face value they're 100% BR, but they're not soul related, which is a weird thing that also comes up in things like OnK and most people disregard it, but I have to say for me personally it does put a bit of distance between the characters, like Karen is more or a legit imouto than Tsukihi because of it.

If you assume it's a metaphor for a child born from an affair, that would make Tsukihi Koyomi's half sister, which weirdly enough feels more legit.

Bee best imouto btw.