In the backwards villages in central Nilfenbergia, feuds between siblings are quashed quickly, for fear of a Rivalry. Once both siblings have committed injury to the other, they are both open to the influence of a Rivalry, an intangible spirit of ill will made manifest.
The first sign is the sudden cessation of hostilities between the siblings. Indeed, they grow close, often being seen conversing together. This is no agreement however, but the influence of the Rivalry, driving the pain and hate of mutual wounds outwards, directed to the village at large. The methods the siblings take depend on their genders.
Two brothers will become sly, manipulative, twisting the desires and dreams of others to crush them and see them hurt, eventually aiming to drive the village apart from each other, a community at war the final goal.
Two sisters will become violent, aggressive, starting physical confrontations and always escalating to naked steel as quickly as possible. Some of the worst bandits of the Nilfenberg Empire have been these sisters, driven by a Rivalry to terrorize, plunder and burn across the realm.
A brother and a sister will be a bizarre combination of the two, both freely switching between roles as the situation determines. A sickening sexual element evolves from this Rivalry, each delighting in the flesh of the other and both in their victims. Kinfucker is one of the more heinous insults in the Nilfenbergian lexicon.
In the case of twins, each gains the ability to shift their appearance to the other, one of them becoming the known destroyer, the other angelic, a seeming force for good in the community.
Banishing the Rivalry is no easy task - one must investigate the feud from it's start, and unravel each wrong committed, finally tricking the siblings into forgiving one another. Records on this succeeding are scant and likely untrue, the Church usually burning them alive once caught.