"Until a month ago we did not know where the siblings were," Omar bin Laden, 29, the fourth son of the Al-Qaeda chief, told the Saudi-owned newspaper Asharq Al-Awsat.
Omar bin Laden, who lives in Qatar, said he only learnt his family members were still alive after receiving a call last month from his brother Uthman, who had been lent a mobile phone by a young Iranian.
Saad, 29, Uthman, 25, Fatima, 22, Hamza, 20, and Bakr, 15, along with Hamza's mother Khairiya are under house arrest in Tehran, Omar said, while a sixth sibling, Iman, 17, recently sought asylum at the Saudi embassy in Tehran.
Omar's brother Saad was rumoured to have been killed in a US drone strike 18 months ago in Pakistan.
British newspaper The Times reported 11 of bin Laden's grandchildren were also living in the high-security compound outside Tehran.
The group fled Afghanistan just before the September 11, 2001 attacks on the United States and walked to the Iranian border, where they were detained and taken to the walled compound by guards, the Times said.
"The Iranian government did not know what to do with this large group of people whom nobody else wanted, so they just kept them safe. For that we owe them much gratitude," Omar said.
His relatives are said to be living as normal a life as possible, cooking meals, watching television and reading, but they are allowed out on shopping trips only rarely.
The Times said Iman escaped from the compound during one such trip last month and fled to the Saudi embassy.
"Iman has lived for more than 25 days inside the Saudi embassy," Omar said.
Fuad Qassas, the Saudi charge d'affairs at the kingdom's embassy in Tehran, confirmed Iman was staying there, according to Asharq Al-Awsat, which said she has been seeking permission to leave Iran, so far without success.
Omar said five of his father's other children are currently in Saudi Arabia, while three, along with his mother Najwa, are in Syria. He hopes the family in Iran will be granted permission to leave.
"We just want to be together as a family. I have now got 11 nieces and nephews, born either in Afghanistan or Iran that I have never seen. Some people may find this story unnerving, but the child can't be judged by the sins of their father," the Times quoted him as saying.
Iran's foreign ministry spokesman Ramin Mehmanparast did not deny that six members of the bin Laden family were in Tehran, at a weekly press conference on Tuesday.
Osama bin Laden, now in his 50s and rumoured to be in poor health, is the world's most-wanted man with 25 million dollars on his head. He is thought to be hiding out in mountains on the Afghanistan-Pakistan border.
