Okay, I don't know if this was already discussed at one point but I really have gut-feeling that Kaname and Yatsude know each other/were a thing at some point in the past.
CHapter 39: Kaname made a 180° turn + her expression after the older twin showed what club shes talking about. Pretty sus.
Chapter 77: Yatsude being all mysterious about know a similar situation like our main pair.
Chaper 82: Kaname talks about her preferences. Someone who looks stern and angry alot. Yatsude open-eye expression is the perfect example of this.