Abu Rumaysah, who was born as Siddhartha Dhar, reportedly fled the United Kingdom while on bail in September, the day after being arrested in London with radical cleric Anjem Choudary.
"My 'kind and generous' brother had changed beyond all recognition since converting to Islam and being radicalised, and laid much blame at Choudary's door," said Konika Dhar, sister of a 31 year-old British jihadi fighting with IS in Syria.
"I believe that being in prison would be better than where he is now and I want him to realise that as well because his perception of the repercussion is slightly distorted and not accurate and things will be okay if he just speaks to us,” Konika was quoted as saying to ITV news.
Dhar was well-known in the UK for frequent television appearances declaring his wish to live with the IS. "I'm sorry if you think Choudary is your role model because he's not," she said, as a message to her brother whom she considers 'kind and generous'.
Choudary was arrested in September with eight others, including Dhar, as part of an investigation into alleged support of the banned extremist group al-Muhajiroun.
Image: Suspected Islamic State militant Abu Rumaysah aka Siddartha Dhar has posted a picture of himself recently on Twitter holding his baby son in one hand and an AK-47 assault rifle in the other. Photograph: Twitter