D.C.
Are you sure he doesn't just assume that you know? My siblings and I made this mistake for a while when we were all young adults. I would tell my mom something, and assumed she would spread the word to my siblings because she talked to all of us most frequently. But, sometimes she would forget to tell them, and we (the siblings) had some misunderstandings. We had to get past our childhood habit of assuming that mom was the central organizer/coordinator. Your brother might still be in this habit.
That said, have you left him a voicemail about graduation? You should at least do that, and not just show up. It might be that he only gets 2 tickets to the graduation and he invited your parents so you actually can't attend. So he's not telling you, because he feels awkward saying "hey, I'm graduating and mom and dad are coming in, but you can't come because I don't have enough tickets". But you could call him, leave him a message saying congrats you heard about the graduation, and you'd love to take him to dinner when finals are over to celebrate.
If he doesn't respond at all to that message - well, then maybe there is an underlying issue that you aren't aware of. But I wouldn't jump to this assumption first.