I can call my brother ugly but you can't.
Interracial crime makes it much more polemic. Our race can murder our children, but we'll be dammed if yours can!
Same with crimes by foreigners. Our citizens can rape our women, but we'll be dammed if yours can!
would cover the bases I think. If one's related to or similar to someone in some way, their anger or dismay at the way that person is abused or treated is meted out in measures relative to how closely related they are.
If they're not related - it's less of a problem for them. If they are related - they go through the roof.
If they're not related - defense is uncertain or maybe nonexistent. If they are related - defense is usually immediate.
Maybe adding the word 'Shared' at the beginning would make it clearer. Yeah, I think Shared Cultural Relativity
would be more precise and make the meaning clearer and more transparent without need for further explanation.