I can see Yamada finally snapping in the next chapter or so and declaring out loud that Kase-san is her girlfriend. Knowing her she'd probably shout it out without thinking and in front of the other girls, only to realize what's happening and overload. That'd be the point I expect Kase-san to save her by being suave and confirming it.
I suspect sempai sees Kase-san as the rival she always wants to compete with, and doesn't want to share any of that person, rather than a romantic interest. Indeed, I'm doubting even more that she and Kase-san even dated, although she might have been interested in doing so