Be explicit. When you hold your own "I" in check, help him notice: "Waiting on this line is so annoying. I feel like yelling at the lady behind the counter, but she's doing her best. Besides, I'm not the only one who has to wait."
Applying for college is very stressful -- not just for your sister, but also for your mom and dad. If they're like most parents I've met, who spend a lot of time and energy helping their kids, it might feel as if they are being judged.
Try not to typecast your boys. View them through a family lens: Your firstborn had you and his dad all to himself for more than half his life. Your second child came home to an already-formed household.