Mourad Laachraoui said there had been no contact between the pair since his older brother left Belgium for Syria more than two years ago.
Najim Laachraoui has been identified by police sources as the second suicide bomber at Brussels airport on Tuesday, and was the suspected bomb-maker for the November carnage in Paris in which 130 people died.
"Mourad Laachraoui firmly condemns the actions of his older brother and the attacks in which he was involved in France and in Belgium," a statement said.
At least 11 people were killed in the attack on Brussels airport. Shortly afterwards a bomb attack on the Brussels metro left around 20 more people dead.
Investigators have found traces of Laachraoui's DNA on explosives used in the Paris gun and suicide bomb assaults, including at the Bataclan rock venue where 90 people died.
DNA traces were also found in a rural Belgian hideout used on the eve of the Paris attacks, as well as in a suspected bomb factory in the Schaarbeek district of Brussels.
Laachraoui went to Syria in September 2013 in one of the first waves of jihadists to leave Belgium for the war-torn country, where he fought under the Islamic State nom de guerre Abu Idriss, according to media reports.
In February, a Belgian court convicted him in absentia for his involvement with IS.
He was known to have returned to Europe in September when he was checked by police under a false identity in a Mercedes driven by Abdeslam, who now sits behind bars in Belgium.
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