Man, you should study Divine Providence, it might help you on your journey on understanding God's sovereignty.
So yes it is God's will for sin to exist, but that still doesn't make Him the author of it.
Sin is more like a tool used in God's sovereign will, if you understand what I mean. Just as God used the wicked men to crucify Jesus. Yet it was their choice, yet God ordained it.
This is why we cannot comprehend His Will to the fullest my friend.