The sister of jihadi Abu Rumaysah says she will "kill him myself" if he is confirmed as the masked Briton in a new Islamic State video in which five hostages are executed.
Konika Dhar, from north London, said she does not believe her brother, formerly known as Siddhartha Dhar, is the man in the video, but she conceded the voice sounded "a bit like him".
She said: "I don't think it is, personally, to be honest."
She went on: "I examined the features - one eye is smaller than the other, my brother has symmetrical eyes. The eyebrows are bushy and this guy is taller, my brother is shorter and he has got broader shoulders, but he has got stooped shoulders so I don't think it is.
"I think the voice sounds a bit like him, but it could be someone else, but he could have helped with a voiceover, but I have no idea."
She said she has not had contact with her brother in more than a year, saying she "doesn't even know who he is" any more.
She also pleaded with the UK authorities to provide an alternative for those who have been radicalised and travelled abroad, to help them return home.
Ms Dhar said: "If it is him, bloody hell am I shocked? I am going to kill him myself. He is going to come back and I am going to kill him if he has done this.
"I can't believe it. This is just so shocking for me.
"I don't know what the authorities are doing to confirm the identity, but I need to know if it is."