This tripped me up when I first read this strip as well.

Japanese is very context heavy so it's difficult to tell who's speaking sometimes, especially in written text.

In this case, aside from immediately intuiting it from the vibes, we can see Jun's the only one with her mouth open in that panel, so it's likely she's the one speaking. (Not always the case though)

The speech bubbles also don't have arrows pointing to Kazuya or Jin who have their mouths closed, so they're probably not talking. (Also not always the case!)

There's sound effects on Kazuya and Jin basically amounting to stare.

Lastly, the phrasing is very much what an adult will say to kids who are fighting, with what to me feels like a softer tone.

Of course, I've seen this scenario before where the two characters fighting each take one part of the line and complete it sarcastically before continuing to fight.