In an interview with his local paper, the Daily Gazette of Schenectady, in upstate New York, David Kaczynski, said his tax position meant that he would have to find more money.
Mr Kaczynski, a social worker, informed the police of his suspicions about his brother, leading to his arrest after a 20-year manhunt.
Theodore Kaczynski, a gifted mathematician and former university lecturer, was convicted earlier this year on three counts of murder and was sentenced to life imprisonment without parole after a plea bargain meant that he escaped the death penalty.
His trial became a legal tussle between himself, his lawyers and the court, after his legal team recommended that he plead insanity.
The Unabomber had terrorised and mystified America with a letter-bomb campaign directed mostly against university staff. His brother's suspicions that Theodore could be the culprit came after US newspapers printed a lengthy anti-government "manifesto" from the Unabomber.
Similarities between that document and his brother's letters led David Kaczynski to track down his brother and tip off the police to the hut where he lived in the wilds of Montana.
David Kaczynski received the $1m (pounds 620,000) reward promised by the FBI for bringing the Unabomber to justice, but he learnt recently that the money would be subject to tax. The remaining $700,000 or so will reportedly be insufficient to pay all the costs associated with his brother's legal defence and legal advice for himself and his family.
The judiciary committee of the House of Representatives, which is considering the impeachment of President Bill Clinton over his conduct in the Monica Lewinsky affair, heard testimony in closed session yesterday from the lawyer acting for Kathleen Willey, the White House volunteer who accused Mr Clinton of groping her.
The committee has extended its inquiry to consider accusations that the White House may have intimidated or induced witnesses to keep silent about sexual misconduct by the President.Reuse content