Thanks for all the replies.

I think family couseling is a great idea. If I can, I'll try to mention
that to his mom (my aunt) the next time she starts ranting about his
behavior. Although, if they decide to do that, they'll have to hold off
until next year. I forgot that my uncle (the step-father) got a new job
in another state and had to start immediately. It won't be until the
end of this year until the rest of the family moves to the new state
too. So the step-father is in one state and the mom and two kids are in
another. Interestingly, my cousin's behavior has gotten even worse
since the step-father left for the new job. I would have thought that
his behavior would've improved since he'll get a bit more of his
mother's attention now. I just hope everything turns out ok for them
all.