Shirin Ebadi, the Nobel peace laureate, has told Al Jazeera that her sister was released from prison in Iran.
Noushin Ebadi was released on Thursday after being arrested during opposition protests at the end of last year, her sister said on Al Jazeera's Frost Over the World programme.
Noushin had been arrested at her home in Tehran in an attempt to intimidate her, Shirin said.
"The government arrested my sister and held her in prison in horrible conditions," Shirin, a human-rights activist and former judge in Iran, said.
"They thought that by doing so they could keep me quiet.
"After three weeks they realized that there was no point and I wasn't going to stay quiet. So they let her go."
Noushin was arrested during days of protests in opposition to the Iranian government from December 27.
At least eight people were killed in the demonstrations which Tehran said were instigated by its foreign enemies - in particular the US, Israel and the UK.
The protests, which led to street battles between demonstrators and police, coincided with Ashoura, Shia Islam's holiest event.
They follow unrest which began after presidential elections last June, which many have said were stolen by President Mahmoud Ahmadinejad.