Sin is a religious term for the violation of moral law or rules. This covers evil deeds committed by conscious beings, as well as some acts (thoughts, etc.) that would otherwise seem ethical in a non-religious context.
Since atheists do not believe in God, it is meaningless to accuse an atheist of being a sinner (or to threaten them with the purported consequences of being a sinner). Humanistic atheists recognize the concept of right and wrong actions in terms of their impact on other people, but the idea of sin is basically irrelevant without appealing to a deity.