The sibling rivalry. Cain feeling like he was treated unfairly, that his brother was the favored one even by God. So often there is jealousy and resentment between brothers and sisters and stepbrothers and sisters over the perceived favoritism of the parents. No matter how hard the parents try to be fair, the perception of unfairness is there. If parents treat all their kids the same, same rules and benefits for everyone.
Every kid gets the same of everything.
Then some kids feel like it is unfair because kids are different and they like different things. So one is going to like what he gets and even though the other gets the same thing, he may not like it. So even though that got the exact same thing, one feels treated unfairly. If parents recognize that kids are different and try to give to them things they think they would like, siblings are always adding up the cost and comparing the value and feeling cheated because they did not get the same thing. So parents can t win this one.