Beep-beep. Lets back up that truck. If Sin is a human concept, then God is also a human concept
OUR concept of what WE think God is, yes, that is a given.
That doesn't change WHAT God truly is though.
Our notion of God and what God "IS" is based, in our case working from the Judeo-Christian background, on the OT and NT and other assorted works.
I don't think that anyone will argue that, IF God exists, what we KNOW of God is very small and incomplete, agreed?
We have a concept, a view, a HOPE of how we WANT God to be and, going by the Bible, how the writers of the bible THOUGHT God was/is, but the bible aslo makes clear that God is beyond our understanding, as God makes clear to Job in that charming little exchange.
Many things written about God are just man's views and as we see with the changing of time, that view changed, hence the seemingly contridictory nature of YHWH and Our Father of the NT, hence the coming of Jesus, hence even the changing views about death and what happens from the OT to the NT.
Point is, the Question is CAN God sin and based on what WE THINK of Sin and what we think of God, that answer lies there.