Although originally thought of as God's favorite, Cain revealed that Abel was in fact talking to Lucifer. In an effort to protect his brother from corruption, Cain offered a deal. Cain offered himself to Lucifer so that Abel would go to Heaven instead. Lucifer agreed, on the condition that Cain himself would send his brother to Heaven. Thus, Abel was struck down by Cain using the First Blade. Abel was taken by Heaven, while his older brother became the most terrible demon to walk the earth.
This act of killing between brothers would set the precedent of their descendants being the vessels of Michael and Lucifer, and thus, again require a brother killing a brother during the Apocalypse.
Cain would eventually confess the true circumstances of his act of killing Abel to their direct decendent Dean Winchester and demon Crowley, reflecting once again how much Sam and Dean's relationship not only reflected Michael and Lucifer's but also Cain and Abel's.