Where did sin come from?
That goes a step back. God is the singularity that creates. Man does not, cannot create. Man can only rearrange. But man’s purpose is to label that which has been created. Forbidden only from labeling or describing God. Another way of putting it is that man creates *abstracts* to describe what God has made.
Mathematics, writing, art, music, theory, all are abstracts.
Prior to eating the fruit, Adam and Eve had no description of good and evil, only that they had been ordered to not eat that fruit. Apparently the serpent *did* have the knowledge of good and evil, so understood what sin was.
So there being no other characters around, the serpent was the first sinner.