"Paramjit Singh is not getting the medical help he needs. He is only getting medical check-ups," his lawyer, Louis Angeles, told the BBC.
Mr Singh has regularly visited India without immigration issues, his niece Kiran Virk told the BBC. This time, his family waited seven hours at Chicago airport for his arrival.
Ms Virk says immigration officials told them Mr Singh was detained over a 1999 case. He was held at the airport for five days despite family appeals, before being moved to a Clay County detention centre in Indiana.
The case involves Mr Singh using a public phone without paying. Court records show he served 10 days in prison and paid a $4,137.50 fine. The conviction has blocked his US citizenship.
Ms Virk alleges immigration authorities said at a court hearing that Mr Singh still faced a one-and-a-half-year sentence, with only 10 days dismissed.
Immigration authorities also say that Mr Singh was convicted of a forgery offence in Illinois in 2008, but his family contends that there are no such charges against him.
Ms Virk said that the authorities cited the forgery case to stay Mr Singh's release on a $10,000 bond granted by an immigration judge.
She said a private detective hired by the family found no criminal records for a person named Paramjit Singh in the state, suggesting authorities may have mistaken him for someone else.
The BBC has asked ICE for a response to the family's claim that there is no forgery case against Mr Singh in Illinois.
Mr Singh's lawyer told the BBC he plans to challenge the detention, calling it "unethical".
"We are also taking legal steps to block him from being deported from the US," Mr Angeles told the BBC.
Meanwhile, Mr Singh's family is increasingly worried about his health, as his second brain tumour surgery has been delayed due to detention, Ms Virk says.
She says the family struggles to contact Mr Singh at the detention centre, where limited phones and his poor health make communication difficult.
Mr Singh's case is set to be heard on 14 October.
His detention comes amid a wider crackdown by US President Donald Trump's administration on immigration, and especially illegal immigrants in the US.
Trump has said he wants to deport the "worst of the worst", but critics say immigrants without criminal records who follow due process have also been targeted.
In September, Harjit Kaur, a 73-year-old grandmother who spent more than three decades living in the US was deported to India, sparking anger among the Sikh community.