AIO For being upset at my girlfriend for laughing with her ex after our first intimate moment

For context were high schoolers (senior (me) junior (her) and at this point we’ve been dating for around 4 months and her ex is her brothers best friend.
After our first time having sex we went back to her house which her brothers best friend (ex) was also there playing bored games with her family. We promptly joined because her parents asked us to. She ended up sitting next to her brother so I sat next to her ex so they weren’t next to each-other. The whole game they were talking so much, showing each-other tiktoks and laughing. All while I was basically being ignored. She then asked me what was wrong and I said what’s wrong is every time I try to talk to you it’s short and you seem more interested in him. To which she responded are you sure it’s that? I feel uneasy and that you might of used me for my body which is why you’re being silent and not talking. We’ve talked about this a couple times to each of which she says “it’s my brothers best friend and he’s always around what do you want me to do” to which I decided wasn’t worth fighting and just let it go. Every time I bring it up it’s the same thing and it seems like it’s actually her inviting him to events not her brother.