In psychology, the study of differences between siblings can sometimes lead to amazing revelations into how disorders affect different people, well, differently. I remember several doctors studying how my Asperger’s affected my younger brother’s mood, and it turned out that, because I got a lot of attention, he felt left out.
There are a lot of common themes between different groupings of oldest and youngest siblings. Some believe that, when there are two siblings, the youngest sibling is evil while the older sibling is good. This is not true. Rather, the oldest sibling is usually the sibling that stays at home, following the rules of the father to the letter. The youngest sibling, instead of being evil, is actually a rebel.
Two examples of this exist in Supernatural. Lucifer, the “younger” of the two main archangels, rebelled from God when he was asked to love the imperfect human creations. Michael, the “older” of the two, decided to be, as Michael puts it in “Swan Song,” the good son, following the rules God set up at the beginning of creation. This included the blueprint that supposedly would lead to the Apocalypse. Lucifer is evil, yes, but in terms of younger versus older sibling, he’s simply questioning the Father’s ideals.
This mirrors Sam and Dean Winchester’s early relationship with their father. Dean, the older sibling, follows father John’s goal to hunt and kill demons. Sam, on the other hand, rebels, choosing to live a normal life until demons kill his girlfriend. Sam isn’t evil, and Dean is not good. Rather, Sam rebelled from the world of the father, while Dean followed it. Neither of them were wrong, just different.
Star Trek: The Next Generation’s Jean-Luc Picard, like Sam, decided to rebel against the rules of his father, while older brother Robert followed the rules. The Picard patriarch, Maurice, wanted both children to stay on Earth and tend to the family vineyard. While Robert listened, Jean-Luc went into Starfleet and won a crap ton of awards. Jean-Luc was the rebel, yes, but “Family” showed that Robert had the jealous tendencies that are usually associated with evil. Again though, neither one of them is evil. Rather, they’re different. And when Jean-Luc needed Robert to help him out, they got into a fist fight. That’s love.
There is always that exception, though, and that exception is Thor’s Loki. Loki is the adopted son of Odin and evil as hell. Because Asgard values strength and not sorcery, Loki got the shaft really early in life. He was doomed to constantly suffer from comparisons to his brother Thor, which really is just garbage. I mean, yes, Loki isn’t an Asgardian, but he is Odin’s son. He should get some love. Instead he gets pushed towards rebellion. Evil? Sure. But for good reasons.
So, to conclude, calling younger siblings evil in fiction is just too easy. There are usually reasons for their evil nature, and it is the combination of several life events that change the character. Don’t make it easy for writers by allowing “younger=evil” to reign.