Well, that is a tough one. First thing that came to mind is how old is this child? It sounds like he is not school age yet. Maybe there is a bedroom near the main areas that can be accommodated for him: lower stimulation, TV with his favorite videos, some toys he enjoys. That way when he gets into something, or gets over-stimulated, he can be redirected in the bedroom. What physical outlet is available? Trampoline, running space. If he is very active this will help. Where is a near park that he can be taken to during part of the day???
I do not have a child with AU, but I have worked with them as a special educator, and it is tough. I would hope the parents would anticipate the child's needs and help to arrange that. If that means moving a few decorations, I would do it. Nothing over the top but within reason. This child can not help his responses to his environmental stimuli....if his parents do not decorate at home, imagine the overload of coming into a fully decorated house. In my heart, the child would come before the decorations. He is a gift just as any other child, and although his parents may not be making the best decisions for him, it again is not the child's fault. I would be inclined to let my husband handle the arrangements since it is his mom and brother; maybe that would reduce the tension. All you can do is anticipate as much as you can; firm behavior plans are not going to be effective for just part of a day. Maybe some visual cues, like a stop sign, placed on doors he should not open, etc. may help--just print off computer or something. Maybe an older child can help 'buddy' with him to let the parents know when something is going wrong. Otherwise, continue what the parents have in place, accommodate him as much as possible, and try to bless him and that family as much as you can.