I act rude when I'm a guest at my relative's house so they won't invite me back.
You're joking right? Why would you push your family away like that?
I like to act like a child when I'm around my cousins, even though I'm too old and it annoys them.
You should probably address the issue directly with your family. Avoiding it like this will only cause more problems.
Don't tell me what to do, I'll do what I want.
I'm sure it can be fun to play with your cousins and it is good to remain youthful but you should also respect the other members of your family, who you are annoying.  There is truly nothing wrong with having fun with your younger family members, but your family also probably wants to see you grow up a bit and it might be good for you.