Secular thought on morality is generally based on negative consequences to self or others where as religious thought on sin is generally based on whether it's offensive to god. Not everyone agrees on what is sinful but sin is a religious thing, generally Christian since even religions like Buddhism don't have "sin".
the secular and religious do not have to be diametrically opposed.
sin is considered negative, or bad behavior. even buddhism has ideas on bad behavior.
you're fixated on a word, a form, at the expense of the idea. ideas and actions aren't exclusive to a belief system.
and no, sin in the religious sense is against a brother, sister, neighbor and in so doing it is still a sin against god. if there is this God that is omnipresent, then God is omnipresent in the brother, the sister, neighbor foremost and not secondary, or elsewhere.
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