Mourad Laachraoui said he had been levelled upon learning his brother was believed to be one of the suicide bombers who had killed 31 people and wounded some 300 in the Belgian capital.
"I was moved and stunned. I didn't want to believe it was him, but you can't choose your family," he told a press conference.
Mourad Laachraoui said there had been no contact between the pair since his older brother left Belgium for Syria more than two years ago.
He described a fairly normal young man and practising Muslim, who had shocked the family by descending into Islamic radicalisation.
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," according to a statement he released earlier on Thursday.
He represented Belgium at the last world championships, which were held in May 2015 in Russia. In July he won a silver medal in the Universiades, an international university-level competition that took place in South Korea.
Najim Laachraoui, 24, was captured on CCTV footage at the airport with Ibrahim El Bakraoui and another assailant who is the subject of a massive manhunt after his bomb did not go off and he fled the scene.