Truth and Justice:
You state that your brother is an elder. Elders have special dispensation to speak to disfellowshipped persons. That doesn't include social contact, but elders are even required to, once a year, contact disfellowshipped persons who were once associated with their congregation to ask if they're ready to take steps to come back to the fold. It sounds like your conversation with your brother was more of a social call, but he can probably justify it under the existing rules. I guess WTS thinks that elders are better able to resist the siren call of apostates and disfellowshipped ones than the rank and file are.