How do I deal with this kind of person

So, my cousin made a group chat for all of us, cousins, and we'd often talk about random things in that gc. However this same cousin often replies like she really doesn't want to talk to us at all. I even joked that she needs to use tone indicators bc I don't know if her replies are being sarcastic or if she's really pissed off at me. It came to a point where me and my other cousins are talking in korean and she replied something along these lines: ''good thing I don't understand what you're saying. Maybe you two are already cussing me out or something" and then she continued to reply with just "ok" even on messages that's not pertaining to her. I'm kinda pissed and also concerned bc I think she's developing some kind of attitude that may or may not affect her gravely once she grows up. (keep in mind that my cousin is an 11-year-old kid)