• When a firstborn’s world changes from being an only child to a sibling and they are close enough in age, the natural feeling will actually be to murder their younger sibling (in some way).
• It is hard for younger siblings to understand this because they are born into a structure that doesn’t change but for the older sibling it is very much an existential threat. All the attention and resources that was once theirs now has to be shared. Given that there’s no conception of ‘enough’ anything less is a massive risk.
• Sibling rivalry is a deep part of folklore and religion. In the Old Testament, Cain killed his younger brother Abel when God preferred Abel’s sacrifice over his. In some variations of Snow White, it is the older sisters who are threatened by the beauty of the youngest.

Notes:

• Sibling Rivalry – Wikipedia