River isn't worried. Considering she rarely gets shot, anyway, and that she highly doubts Simon will go through with that threat, even if it's Jayne who gets shot next.

In fact, she completely ignores Simon to fix a wide-eyed gaze on Kaylee, realizing that something about the unfamiliarity is a little familiar.

There's a question in that look, but I wouldn't even try to put it into words.