Paul Cantlie, 80, recorded a video message from his hospital bed earlier this month, urging those holding John Cantlie to release him.
A family statement said Paul Cantlie had died from “complications following pneumonia” on October 16.
Journalist John Cantlie, who was kidnapped in Syria in 2012, has appeared in a series of ISIS videos.
In a statement, Paul Cantlie’s family said: “Paul died not knowing whether John’s captors had received any of the messages he had sent to them.
“The failure of communication to date has inevitably led to a terrible sense of abandonment, particularly for John’s father, with the family feeling ignored.”
The statement also said the family had been urgently trying to contact John Cantlie in the last few days to let him know of his father’s death ahead of the news being made public.
“Many in the country will recall the recent broadcast from his [Paul’s] hospital bed, when so demonstrably ill, but determined that his journalist son John should hear from his father <<how very proud I am of him>>,” the family said.
Since August, ISIS has killed four Western hostages – US journalists James Foley and Steven Sotloff and British aid workers David Haines and Alan Henning.
The group, also known as ISIL, has taken control of large areas of Syria and Iraq and declared a caliphate.
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