What to do with a bad 6 year old?

Basically, I came over for half the summer as a babysitter to my aunt while her husband is away. She has two kids who I have to take care of, my cousins. One of them is almost an angel with the exception of when he starts cursing at the younger one. However, the younger one, 6 year old, has this annoying attitude that my aunt is begging me to help her solve. What happens is that he doesn't like to listen to anyone. My aunt threatened to send him to the camp (he hates it) if he causes me trouble but he always pulls the "this is my house and you're not the boss of me". If I threaten to do anything to him, he goes/calls to his mother who in turn yells (she tried convincing calmly) at him for misbehaving. Whenever she tells him something, he puts on a funny face and ignores her. The thing with this kid is that we tried everything from to locking him out, but it doesn't have any effect on him. He doesn't get angry and refuse to talk to anyone, he just goes back and (cont.)