Issam Abuanza's family "will never forgive him", his sister Najla has said, following reports that the 37-year-old deserted his wife and two children in 2014 to join the militant group.
The revelation was unearthed in Isis recruitment documents leaked to the BBC.
Abuanza's sister told the BBC: "He used to be quite the dashing young man, very modern. I've no idea how he became like this or who showed him the path to terror.
"My dad's wish was to see him before he dies.
"He has spent all his money on him and his education and this is what he does."
Abuanza, who lived in Sheffield, gained his medical licence in 2009 and is believed to be the first NHS doctor known to have joined Isis.
He posted "God bless this act of terrorism" on his Facebook page on the day of the Charlie Hebdo attacks in Paris on January 7 last year.
The attacks at the headquarters of the satirical magazine saw 12 staff members gunned down by two Islamists shouting "Allahu akbar".
Abuanza described the US as "godless" in another post made on the social media site.
One photo posted appears to show him reading the Koran while cradling an automatic rifle.
Another apparently shows him posing in doctors' scrubs with a pistol holster slung over one arm.
Born in Palestine, Abuanza spent seven years working for the NHS and is believed to have worked at Scarborough hospital between October 2012 and August 2013.
Scarborough and Whitby MP Robert Goodwill told the Press Association: "It's shocking that someone who has worked to save lives in the NHS is now promoting one of the most murderous and horrific regimes.
"We read about how young Muslim men are being indoctrinated by the Islamic State but this man was a qualified doctor, proving that even those who are very intelligent can be hoodwinked."
The Transport Minister stopped short from speculating that Abuanza could be among many other professionals to have joined IS.
"I hope not, the majority of Muslim people I have met are very sensible," he added. "This is a perversion of their religion."
Goodwill does not recognise Abuanza's name but says they could have crossed paths when he visited Friday prayers held at the hospital.