Friend of mine is married with an american and have a kid with 3 years that born in Brazil. What he need to do to legalize the kid in USA?

See this, as a start:

http://travel.state.gov/family/adoption/info/info_448.html

I would certainly advise the parents, though, to seek legal advice on an issue as important as this.

Zarathustra

That's good. That means my step-siblings are already citizens.

I thought they were automatically, just wasn't sure.

As I read it, if they meet the conditions specified, they are, Sphinx. However, there may additions to that law or changes since then, so my advice is to get legal advice. As a precaution, I would also get them US Passports.
Cheers,
John