Thinking more, I feel like the real worry here is not how onee-chan handles it (she'll probably at least play it cool enough while the call is still going and save the teasing until afterwards) but rather that Kyou "SHE'S NOT MY GIRLFRIEND"s a little too hard and hurts Yamada's feelings (or maybe the other way around, but that's far less likely.)
@Gravewind the call being so sudden followed by the glasses+cleavage combination caused him to let his guard down.