Haven't read the whole thing yet, but...
Sorry, this kid belongs in a mental institution... in his current condition, he is a ticking bomb... a tragedy waiting to happen. I understand his mother wants the best for him and to have a normal life, but somethings aren't meant to be.
Under those circumstances, I couldn't keep a child like that under my roof, when he has already said he will hurt the other children, and you that is a real possibility. If no other children were at risk, perhaps, but I think it is irresponsible putting all his siblings and other family members at risk.