So does sin exist or not?
Yes...but only for people who accept that a GOD exists...and that the GOD reveals what pleases and offends IT.
"Sin" is anything that offends that GOD.
So whether the GOD actually exists or not is not essential to the thrust of the question. If a person accepts that the GOD exists...and accepts that the GOD has indicated what offends IT...then anything that does offend IT is a sin.
The question that arises in my mind is: Why would anybody stoop so low as to worship a god that gets offended by as much insignificant nonsense as offends, for instance, that god of the Bible?