Of course, you cannot allow your children to be physically injured whether or not someone is autistic. There are only a few limits, and physical security is one of them. It’s doing everyone a disservice to allow your nephew to hurt people, so start there. Arrange a meeting with your in-laws or siblings and let them know that this violence needs to be dealt with for both of your families. It is unfair to continue to subject your nephew to situations that make him feel unsafe and deregulated, so what else can be done? Typical triggers for many children with autism are sensory, so can your nephew wear headphones for the first few days he’s with you? Can he be in the house, but in another room (if he wishes)? Can you eliminate loud noises, strange smells or new objects? Is there access to exercise, a trampoline, a swing, or some other form of movement that might calm your nephew?