An eyewitness who watched two sisters dart into M6 traffic before one of them fatally stabbed a Good Samaritan says he thought they had "superhuman strength". The bizarre case of Ursula and Sabina Eriksson, who baffled police by running onto the major UK motorway in 2008, setting in place a chain of events that led to the gruesome death of a former RAF serviceman, is being revisited in a new Channel 5 documentary.

The two-part series, titled Twisted Sisters: Madness and Manslaughter, will attempt to make sense of what experts have described as a rare psychological phenomenon called "folie a deux", or the madness of two. The Swedish twins arrived in North-west England in May of 2008 after fleeing mum-of-two Sabina's home in Ireland , initially boarding a National Express bus from Liver

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