How do I deal with younger cousins?

I'm around your age, and I have a 12 year old sister, and I have the same kind of authority problem with her. What I've found is you have to be a mix between a friend and an adult, it's hard though. Maybe talk to your cousins and come up with something were you can play with them, but when you tell them to stop, they have to listen. Try not yelling at them, but being firm. I guess if it really doesn't stop, just try avoiding them for a while, but that should be the last resort. Hope this helps.