A sin is a transgression of religious or moral law, and Buddhism doesn’t really have laws. It has the Four Noble Truths and the Ten Precepts – and a multitude of other numbered lists of things to watch out for, but none of them are laws. The Buddha suggested some behaviors which will probably make our lives easier and happier, and make it easier for us to move toward understanding the true nature of reality, but they weren’t strict rules about how to behave. They were just revelations about what would be a skillful means toward enlightenment. One of the Ten Precepts is “don’t tell lies”. If you lie, you’re just making it harder for yourself to become enlightened: its not a sin.