My guess is that Kei's mother had enrolled her in Shuichin early on, so Kaguya was actually Kei's Senpai before Miyuki had even enrolled.Kaguya 208
I loved this chapter for countless reasons - Kei's reactions to the fact that Kaguya and Miyuki shared the same bed for the night, the first poster in Miyuki's room being related to the desire of standing alongside Kaguya, the backstory about Miyuki's mother.
But, among all these things, I found the dialogue between kaguya and Kei extremely interesting for the lore of the series. I don't remember Kaguya giving Kei the advice that wanting some space for themselves doesn't mean to reject the loe of other people. This means that Kaguya and Kei have met in the past, possibly before Kaguya met Miyuki himself, and this might be related also to the identity of Miyuki's mother. Perhaps was/is she working for the Shinomiya family? That would explain how Kaguya could have met Kei, since Kei's mother could have sometimes brought her child to Shinomiya's house.
Then the chapter ends with a cliffhanger. Fortunately there won't be a break next week. If I were Miyuki, I would think that Kaguya is going to break the relationship after finding out the posters (she won't, of course).
Maybe Kaguya had given Kei advice then, but because she was still ice-Kaguya it was such an insignificant memory that she didn't bother to remember.