Thank you JWFacts. Very much appreciate the link, and the advice. You've echoed my thoughts, but it is much more difficult to convince my parents of that. I do realize that the only reason he got involved with JW is because of his girlfriend, and that convincing him to leave it will mean that he will also very likely have to leave her. I realize it makes his choice to stay easier.
One odd thing I've realized as I do more reading here. This girl has gone to university, and she is a teacher by proffession. She has regularly slept over at my parents house (in the same bed as my brother). She has even (although he will not admit it) sort of cheated on him with her ex-boyfriend. Whether the cheating was physical, I don't know. I do know that she hid the fact that she met him for coffee/dinner a few times from him, and they argued about it within earshot of me.