Tell them that you are now an atheist, and you don't believe in "sin"! :twisted:
As John Steinbeck put it through his character John Casey,
"There is no sin, and there is no virtue...there's just stuff people do!"
Yes, I believe this. Good and evil do exist, but they are defined by the consensus of conscientious humanity, not the arbitrary ramblings of prophets who never really saw the world which we live in today. When you have the intelligence to know what's best for you, you should go with it, and nobody else should have the right to say otherwise, so long as you aren't causing yourself and others actual harm. When they do decide what's best for others, that's where the actual harm is committed.
"Sin" truly is the most evil word in any language.
Now onto the question of whether or not to smoke while staying with your parents: you need to weigh what's more important to you. If spending quality, strife-free time with them is important to you, then maybe you don't need the pipes so much, and that depends on whether you believe you can get through the duration of your stay without them. On the other hand, your parents may be so difficult that you need the pipes just to cope with them at all, and in that case guess they'll just have to live with your smoking. Trust me, I've been there!