Genesis 4:7

If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.

1. The LORD reassured Cain that if He yet made an offering that met the LORD's criteria for a good sacrifice he and his sacrifice would be accepted.
2. If we do well the LORD will accept us.
3. Sin was the reason Cain did not make an acceptable offering to the LORD. The nature of his sin is not mentioned.
4. We cannot please the LORD if we continue to sin.
5. Cain was to rule over someone who would look to him as his leader. It doesn't make sense that this would be Abel, since Abel was a righteous man. No other brother is mentioned - only sin is mentioned, but to rule over sin makes no sense either. It might be the serpent who tempted Eve to commit sin, but He isn't mentioned here either. It appears that some of the story has been left out, either by accident or intentionally.