I think that this should be handled by talking to the younger brother first, since he came to the offline casino on his own will and even stole from his sister. Counselling or any professional help is important, because this conversation is as relevant as changing the course of his future decisions. But of course, it cannot be overlooked that the operators ignored a raised concern which deems to be a fault of the management as well for allowing a minor to their charge, which is clearly indicated to their protocols about minor player prohibition. This could be raised upon the attention of those in the authorities like the regulatory boards for gambling, etc.