Tuchak said his office is working to bring the brother back to his native Russia, even though the American parents have not been charged with any crime or publicly named.
"You need to start taking the necessary procedures today," Turchak said. The brother "can't stay in the United States. With American legislation, he would be transferred from one hand to another. It's an additional trauma for the kid. He's not a dog or a cat."
Texas Child Protective Services says its top priority is the surviving child or children at the home, Crimmins said.
"At any time, if we feel the surviving child or children may be at risk, we can remove immediately," he said.
A call seeking comment to a number listed for the boy's adoptive parents was not immediately returned. A voice message told reporters the parents would have no comment.
In December, Russian President Vladimir Putin signed into law the measure that would ban the adoption of Russian children by U.S. families. It is scheduled to take effect in January 2014.
Americans adopted close to 1,000 Russian children last year, according to U.S. State Department figures.
Though the number has been dropping in recent years, Russia remains the third most popular foreign country -- after China and Ethiopia -- for U.S. foreign adoptions.
The Russian measure also bars any political activities by nongovernmental organizations receiving funding from the United States, if such activities could affect Russian interests, Russia's semiofficial RIA Novosti news agency said.
And it imposes sanctions on U.S. officials thought to have violated human rights.
The move is widely seen as retaliation for a bill that U.S. President Barack Obama signed into law on December 14. That law, called the Magnitsky Act, imposes U.S. travel and financial restrictions on human rights abusers in Russia.
The act is named for Russian lawyer Sergei Magnitsky, who uncovered the largest tax fraud in the country's history in the form of rebates claimed by government officials who stole money from the state. Magnitsky was apparently beaten to death in 2009 after a year in a Moscow detention center.
Backers of the Russian bill said American adoptive parents have been abusive, citing what they say are the 20 deaths of adopted Russian children since the 1990s.
The Russian public supported the bill, with 56% of respondents in a poll conducted by the Public Opinion Foundation saying they backed the ban, RIA Novosti reported.
In 2010, an American woman sparked outrage after she sent her adopted son back to Russia alone on a one-way flight, saying the boy, then 7, had violent episodes that made her family fear for its safety.