Tsarni promised Saturday to help his nephew seek forgiveness from the bombing victims and advised him to tell police everything he knows.
The brothers come from a family originally from the Russian republic of Chechnya and fled the brutal wars there in the 1990s. It's unknown how their Chechen roots may have influenced their alleged actions.
Tsarni said he believes Tamerlan Tsarnaev influenced his younger brother.
FBI agents interviewed the elder Tsarnaev in 2011 at the request of a foreign government that suspected he had ties to extremist groups, the FBI said. It declined to name the government, but a senior U.S. official told CNN on Saturday that it was Russia that made the request of the United States.
The request was based on information that Tamerlan Tsarnaev was a follower of radical Islam, the FBI said, adding it found no evidence of terrorism activity.
"I think unless we see some horrible dropping of the ball, I don't think this is an intelligence failure," said former CIA operative Robert Baer. "In retrospect, it might look like one, but I don't think it is."
The suspects' father, who lives in the Russian republic of Dagestan, told CNN on Saturday that he believes his sons were "never, ever" involved in the Boston attacks. He also said he plans to go to the United States, though he didn't say when.