Do not attack, especially if you are uncertain of answers to questions they will make towards you (I know I was, but then again, I wasn't attacking them, just giving my point of view of life.) I personally have had friends become JW, I say, take care of what you say, and do not attack their point of views, and if better yet, don't ask question that could lead to conflict, you do not want to be the bad guy, just be neutral to any question you may ask. However SHOULD they ask you what you believe in, you are screwed if you do not answer that you believe in god. I stood up to my beliefs, and even stated that I wasn't that knowledgeable in science, however they tried to make me believe in their version of god, to which I just basically stated, what make you right out of all the other sects of christianity (hint they believe their interpretation of the bible is right), I then reminded them that EVERY sect believes that, so I will stay in my default position that it's all BS. I was never invited again oddly enough, however this is not what you want to do, do not give them a reason to isolate your sister.