The sister of an Indian-origin Islamic State terror suspect dubbed 'New Jihadi John' today said she is hoping that the masked man in a recent video of the terror group is not her brother and her family was in the dark about how he developed increasingly radical views.

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 IS 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 who 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.

Siddhartha, a former bouncy castle salesman from London, had changed his name to Abu Rumaysah after converting from Hinduism to Islam.

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.