In all personal belief, I would slap that cousin hard! Ain't anyone touching my animals... (Then again, I am not anywhere near 18, or above! LOL) I really do believe that you must talk to your parents, his parents, and the 8 year old as well. Before getting a parrot or even deciding on having my heart set on one, I really discussed it with my parents. I set down rules, educated them on parrot behavior... My dad still needs to understand that parrots can be potty trained, but other than that, we're ready to go! We just need to find the parrot for us.
I think that a lengthy discussion with that 8 year old is what needs to be done. I would sit him down and threaten him. I don't know, kid stuff -- "if you do that again, i'm going to do this to you. Don't tell your parents, got it?" It's not that hard to manipulate an 8 year old. And talking to the parents is another option. But seriously, many people are ignorant. It took A LONG TIME for my parents to understand that parrots aren't broken records that just imitate noises. They're intelligent, living creatures. A pair of people that let their child run around like that are obviously, a) not very educated or b) not intellectuals. Even though my parents (like any other human being) needed to be taught certain things, they wouldn't let me hit or scream at any animal. I was taught not to throw tantrums or hit the walls of animal enclosures waaay younger than 8 years old.