Also, it's time to go on a campaign w/ cousins and aunts to say, "We don't always get info that is sent to us through the parents--they forget which of us they have or haven't told. So you need to ask us directly, with our own invitation or phone call. Do you have all our contact info? Here's our phone number, our email, are you on Facebook?"
And then if you ever miss an event because word was sent through parents, you proactively call the host and say, "Oh, it's too bad, we can't come, we have other plans already--you should have contacted us directly, because DParents are really bad at passing on the info to us. You just can't count on them. Do you have all our contact info? Here's our phone number, our email, are you Facebook?"
If you guys (esp. your DH) can detach from the chastisement ("where are you? Your mother wants you here" may be a scolding but your DH doesn't have to buy into it. He can decide "Fffft! Sorry, Dad, I'm not going to feel guilty--sorry, we're busy. You didn't speak for our time soon enough." You guys can decide: "They don't have the right--or the power--to scold us for not showing up in this circumstance."
It's very powerful! You can't change them. You can only change how you react to them. So turn the "guilt" off.
I'm wondering if the brothers are just physically around more, and are more tightly tied to their parents, and that's why they know about these things when you don't. And that's why the parents forget that they have to speak sooner with you guys.