She will have to petition him with an I130 ( Just did the same thing for my stepson and he arrived on the 9th). Since the child is under 18 you will probably need the other parents permission for the child to travel to the US, unless she has full custody already. She doesn't need citizenship to file as long as she is an LPR she can file for her child. My wife is just an LPR and we had no issues. We also waited and filed about a year before his 18th birthday, that way he was able to get his own passport, and was able to avoid any issues with his father not letting him leave the country. It was no issue, but I wasn't going to risk it. She can file the I130 all online, and it was really easy and straightforward. Should take 14-16 months for the child to have a visa, then child can travel to the US, and apply for citizenship after 5 years on his own. Age is the only factor that would change things. Child has to stay unmarried and be under 21 at the time you apply for the visa. to be safe, better child is under 21 when visa is issued.
On the citizenship question, it all depends on Brazil and if they allow dual citizenship, a quick google search should answer that, but they more than likely do allow dual citizenship.
Looks like your good on the dual citizenship...
https://thebrazilbusiness.com/article/dual-citizenship-in-brazil
It's an older article, so make sure you search for something more current, but usually things like this don't change very often.
Also, guess I should have asked if child is already here in the US, that makes a difference also.