How could they sin when they didn't know what evil was and until they knew what it was?
Eve, didn't act confused at all when she restated what God said. She knew what the law was. She simply chose to believe someone else. And this is the core issue of our entire existence.
You claim sin entered the world through them eating the fruit
The fruit is just a manifestation of the core issue: each person must personally decide whether or not to believe God. The sin resides in the unbelief, just as life eternal resides in believing God.
"He that believeth on the Son hath everlasting life: and he that
believeth not the Son shall not see life" - God
Things haven't changed since Eden, it's still about belief, and we still have a choice.