If hearing voices telling one to hurt others were a common childhood experience for Aspies, it would be mentioned frequently in discussions about childhood here on AC. However, I haven't noticed it in the time I've been on here. That sounds like schizophrenia to me, which can be present alongside ASD.
Psychosis can be experienced more commonly in autistics but I don't think it's clinically considered an autistic trait.
I would suggest to your sister she pursue this further with those "professionals", or even better with some different ones. I think it's unusual for a child to experience this kind of behaviour. It must be awful for him, and I understand how worried your sister must feel.
Definitely read the article that
Judge linked to. Especially the part about implications for psychiatric practice.