Konika Dhar appeared before a House of Commons Home Affairs Committee hearing over the possibility of her brother Siddhartha Dhar being the masked man who recently appeared in an ISIS propaganda video which showed "British spies" being executed.
"I'm still holding to the firm belief that what I'm seeing is not him -- and I haven't had verification otherwise. It's sort of the realisation that 'is he really my brother that has done this? and I can't accept that he would ever do that. I can't accept it," the London-based law student said.
Speaking to the parliamentary committee, his sister said she and her family were "left in the dark" over how he came to convert about a decade ago and how he adopted his radical views, but feared he had been "brainwashed" by individuals in the community.
In response to a question, she said it would "absolutely" be a good idea to have organisations for families to turn to for advice or share their concerns in confidence.
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"I think this is one thing that needs to be addressed, because for me personally it was very difficult to know who to turn to. I didn't know whether to contact the police, whether to go via the media or speak to family members...It was a bit of a shock," Konika said.
Labour MP Chuka Umunna asked her if she felt responsible for her brother.
"I feel a sense of guilt definitely -- I've lost my brother and why was I not able to stop it because he is part of me," she said.
'The Jihadis Next Door', to be aired on Channel 4, captures him telling the filmmaker: "These are the black flags of Islam.
This one's actually the flag of the Islamic State, so one day when the Sharia comes, you will see this black flag everywhere. He skipped bail and fled Britain later that year with his wife and their four children to join ISIS in Syria.