Who is in the wrong here?

Scenario starts like this – a friend of me and my brother from back home is in town for a few days. I text my brother earlier today around 1:15pm (I work til 5pm) asking him if he was going to make plans to hang out with our friend. My brother says “yeah” and doesn’t really elaborate. I texted our friend on and off until about 4:00pm. I didn’t hear anything back from my brother, and my girlfriend asks me if I want to BBQ, so I say sure. My brother makes plans to hang out with our friend and invites another friend of ours as well but doesn’t mention the plans to me (later says he assumed I wouldn’t go as I already made plans to BBQ he found after asking my gf) Should my brother have at least told me about the plans so that I could have at least made a decision, or is it proper to not ask since he knew I had made other plans?