I am not in your shoes, so I do not know your whole situation. But I have been in similiar situations as have others here. I would tell the witness what I felt. I felt the witness as a individual was a good person, but that was not my problem. My problem was with the society. I would show the witness some things that contradict what the society has done such as the UN issue and so I could not trust the society. If they are really persistent in questioning why you do not convert, then do some research. http://quotes.jehovahswitnesses.com is a great place to get watchtower quotes without commentary. Get some quotes from there and show this family this stuff. But ask them questions about this stuff so it gets them to think instead of trying to prove them wrong. Good luck.
You lie so much you believe yourself
Judge not lest ye be judged yourself