"Today Hossein's (blood) pressure dropped severely and he lost consciousness," he said.
Ronaghi has been denied medical leave "despite the fact that he... is in critical condition", his brother added.
"Hossein said he would begin refusing water from tonight."
Ronaghi had been giving an interview to London-based Iran International television on 22 September when security agents came to his home, the channel said.
It published a video showing Ronaghi looking anxious but insisting the interview continue.
The activist, who contributes to The Washington Post, managed to slip out via his building's car park and later issued a video message from an undisclosed location, it added.
But he was detained and beaten by security officers when he went to Tehran's Evin prison to meet prosecutors on 24 September, his brother said at the time.
"Whatever happens to Hossein, the judiciary, intelligence ministry, prisons' organisation, the prosecutor and the leader's office will be responsible," Hassan tweeted on Saturday.
Oslo-based group Iran Human Rights says at least 304 people have been killed in protests across Iran since Amini died in custody on 16 September, three days after her arrest for allegedly flouting the Islamic republic's strict dress code for women.
Thousands of people have been arrested, including more than a dozen lawyers who were detained after working to defend those already in custody.