first off, how old are you (legal age?) second, since you already told them you smoke them but only a little bit say something "sincere" like, "I do smoke everyonce in awhile but seeing how much it bothers you two, I will do my best to not touch another cigarette."

Be honest and they will learn to trust you throughout the rest of your stay with them. talk to them like an adult and be receptive to their complaints. that is all you gotta do to get through this. it kinda is a pain to be all good and shit but hey...