I'm so sorry this has happened and it really does put you in a difficult position. If he's telling the truth and you disbelieve him, it means an underluing problem goes unidentified and in the future others will be at risk. if you believe him, the same thing can happen.
What I suggest (which you're probably already doing) -
1) Get easy child somewhere safe. Also take steps to keep others safe from difficult child's 'sleepwalking'. If tat means keeping everyone else safe at home while difficult child is moved out - so be it. But whil he is at home, it's easier for you to monitor him. If you keep him at home, instal motion detector infra-red cameras to record any nocturnal excursions.
2) Take steps to assess the danger from difficult child. If he is sleepwalking in this way then a sleep disorder clinic needs to do a sleep study on him. He needs to be wired up for eeg while he sleeps, so his brain wave activity can be monitored while he sleeps. He needn't be attacking anyone in his sleep for the disorder that would cause this, to show up.
Failure for anything to ashow up doesn't mean he was lying, but it does weaken his case.
3) If a sleep disorder is diagnosed, then it can be at least known and hopefully treated. This should work to keep people safe iin the future. It could also mend relationship orlbmes that can be caused by his attack/behaviour, if he IS telling the truth. And if he was lying - it calls his bluff and again, lays cards on the table. You know where you stand.
difficult child should not be distressed by any steps taken to get him assessed for sleep disorders, not if he is genuine. ONly if he was making it up, would he object to this. easy child should also welcome this because again, it will clrify what happened and if she believes he was really attacking her knowingly, then it will validate HER.
Hugs from Down Under for this. Not pretty.
Marg