I've said it elsewhere and I'll say it again: YAAAY!
Cass is welcome to be the eldest twin, what with Hamish being the weak, frail one when they were born. I have the feeling this would have encouraged their grandparents to take Cass and leave Hamish behind even more.
I think said popular theory is probably not helped by the fact his brother tends to show his sensitive side more often. No wonder they don't like being compared to each other.
Oh, and Hamish would like to know which of them is meant to be the annoying one. Is this something like the "good/evil" argument?
Alternatively, I should mention that Cormac and Cass were probably brought up in the same social circles, what with the filthy rich Pureblood background, and since Cormac learned of the whole half-Muggle business around the same time he discovered Muggle-borns were not so bad, he would have just shrugged about it. Of course, given that Cass plays for the Slytherin team, Cormac's current opinion of him is a bit tainted by Quidditch prejudices - and also for his attempts to rile up prefects.