Why would Dudley call Harry "cousin"?
If he hated him
At first I was about to say if he said "bro" and "sis" in this movie as well, then this would be a stupid movie, cause it would be promoting that that "bro", "sis" and "cuz" must be black slang because it's related to the slangs "brotha," "sista," and "cousin." But he never uses the slang again, only to refer to Harry and he hates him.
So what's the deal.