Bor Mabil, 19, was killed in the accident in Adelaide just hours before Australia's 1-0 defeat to hosts UAE, in which Awer Mabil was a second-half substitute.
Reports said Awer, 23, was unaware of the tragedy when he took the field. Football Federation Australia said he was receiving support.
"FFA extends its deepest sympathies to Awer, the Mabil family, and Bor's friends, at this difficult time," a statement said.
"FFA is providing support to Awer, his team-mates and Socceroos staff in the United Arab Emirates."
Two drivers have been charged over the accident, in which a car carrying Bor Mabil crashed into a fence, according to Australia's ABC. She reportedly died at the scene.
Awer Mabil's international career has started to take flight at the Asian Cup, where the Denmark-based forward scored two goals and started four of Australia's five games.
The Mabil children grew up in a mud hut in a Kenyan refugee camp after the family fled conflict in Sudan, before resettling in Australia in 2006.
Awer Mabil has grabbed attention for his distinctive goal celebration, placing one hand over his mouth and with two fingers from the other hand pointing at his forehead.
The aim is to raise awareness of mental health issues such as depression, which he has witnessed all over the world, he told AFP earlier in the tournament.
"It's something that I keep in mind because I know a lot of people suffer from it. The sport we play, we're very grateful but it's not easy, it's not what people think it is," Mabil said.
"People suffer from being alone -- you can have all the things in the world but if you don't have peace of mind, what's the point?"