The family members were detained after fleeing from Afghanistan in the aftermath of the September 11, 2001 attacks.
Stuck in Iran
Omar, 28, said his family members cannot leave their compound or make phone calls without permission, and Iran has refused to release them to their native Saudi Arabia.
"Othman called me by phone four days ago and asked me to find a country to mediate their release and accept to receive them," Omar said.
He said his sister Iman was released three months ago, after she took refuge at the Saudi embassy in Tehran, and is now in Syria.
The government of Qatar, where Omar currently lives, has also been working to help relocate the family, he said in a separate interview with the Reuters news agency.
"Iran has a problem with Saudi Arabia. They don't want [the family] to go back home. That's why I am asking for Qatar's help, for Syria's help," he said. "They want to come here, or to Syria, or to any country in the world that would accept them."
Relations between the United States and Iran are frosty at the moment. Iran has railed against a new round of sanctions which were championed by the US and recently passed by the United Nations Security Council.
Iranian scientist Shahram Amiri, who US officials say defected to the United States only to return to Iran this month, has claimedthat he was abducted and tortured by US intelligence agents.
Relations between Sunni Muslim Saudi Arabia and non-Arab, Shia Iran have also been marked by rivalry and mutual mistrust, in part due to Iranian nuclear plans.
Qatar is a close US ally, which hosts a large US military base.
The tiny country, which is the world's top natural gas exporter, has sought to carve out a role as a regional powerbroker, particularly in its efforts to engage Iran.
Doha hosted Mahmoud Ahmadinejad, Iran's president, last year in a crisis meeting over Israel's invasion of Gaza, which was boycotted by Egypt and Saudi Arabia.
Omar said he has also been working with other organisations, including the United Nations, to obtain his family's release.
He broke with his father in 2001 after living in Afghanistan for much of 1996 through 2001. His father is believed to have about 20 children from several wives.
Omar left Afghanistan several months before the September 11 attacks on the United States. He has been denied an entry visa to the United Kingdom, and both Egypt and Spain have rejected his asylum requests.