He has visited his brother in Batam more than 10 times since his brother was arrested in October last year.
Singaporean businessman Lim Yong Nam, 41, is detained at the police headquarters of the Riau Archipelago on the Indonesian island.
He is wanted by the US for allegedly breaching a US trade embargo against Iran.
His younger brother, who wanted to be known only as Mr Lim R.Q, 37, told The New Paper on Wednesday that he feels very sad to see the state his brother is in there.
"I think Yong Nam has lost more than 10kg and is now skinny.
In just six months, he looks like he has aged by 10 years. He was coughing the last time I saw him in mid-March.
"He looks stressed, tired and doesn't look like my brother any more," said Mr Lim R.Q, who works in sales. "He always tells me to keep an eye on his family if anything happens to him in Batam."
The US accused Lim of acquiring 6,000 radio frequency modules for export to Iran and asked for his extradition in 2011.
But he was not extradited as the Singapore High Court found that the wrongdoing he was accused of was not an offence here.
On Monday, a Batam court ordered Mr Lim's release from detention after it found that the Riau Archipelago police did not have sufficient documentation at the time of his arrest to detain him.
But he was rearrested just after he was released from detention the next day.
Mr Lim R.Q said that their elderly mother, who lives with him, was looking forward to his brother's return and was extremely disappointed when she discovered the outcome.
He said: "She was so happy at first that she even told me that she dreamt that he was back in Singapore. She looked very disappointed and came down with a fever when I told her that he was not coming home.
"I'm worried for her as she was treated at IMH (Institute of Mental Health) several years ago for depression after she was diagnosed with breast cancer."
He added that he was at work on Tuesday when his brother's wife, Ms May Lim, 40, sent him an SMS with the bad news.
After her husband's arrest last year, the mother of two young daughters sold their three-bedroom apartment to pay the lawyers' fees. They are now living with her parents.
Said Mr Lim R.Q: "On Monday, I went on the Internet and was very happy when I read about the Batam court's decision. But at the back of my mind, I felt that I shouldn't celebrate too early - not until Yong Nam is back in Singapore.
"When May sent me the text message on Tuesday, my worst fears came true. I'm very sad, disappointed and angry.
"Batam police told my family that he will be held in remand until his extradition trial in two weeks' time."
Lieutenant-Colonel Armaini of the Riau Islands police had earlier told The Straits Times that Mr Lim was rearrested as they feared he would flee before his extradition proceedings could commence.
Responding to queries from TNP, the Ministry of Foreign Affairs (MFA) said that it called in the Indonesian Charge d'Affaires in Singapore, Mr Ridwan Hassan, yesterday to express its concern over Mr Lim's continued detention despite the Batam court's ruling.
The MFA added that it has been closely monitoring his case since he was first detained and has cautioned that Singaporeans travelling in foreign countries must abide by and are subjected to the laws there.
A spokesman said: "We have requested Indonesia to urgently provide an explanation on the legal basis for Mr Lim's re-arrest and detention.''