I think sin is comparable to what crime can be to a society. Therefore I do not think sin is only applicable to religion. But I think religion has defined it in such a way there really is no judge. The judges in a religious context (i.e. priests or church leaders) cannot really say whether one is at fault and "sinful", because sin is a matter of the heart and spirit. And no one can really look inside someone's soul.

Another fault with the belief in certain things being "sinful" and others not being "sinful" is that everyone belonging to a religious group or spiritual group (if they dare to claim a relationship with God rather than a church/group) have varying beliefs on what is "sinful" or not. Also, circumstances can define what can be sinful.

I think that the only real sin is if you are vindictive to someone or yourself. In that, you are failing to respect yourself, others and whatever spiritual being you associate with.