* Different origins: Sonic was born naturally on Mobius, while Shadow was created by the mad scientist, Professor Gerald Robotnik, as an ultimate life form.
* Genetic connection: Shadow is essentially a genetic clone of Sonic, created using Sonic's DNA and the power of the Chaos Emeralds. This makes them more like biological siblings.
* Conflicting ideologies: Shadow was initially programmed to be the ultimate weapon, leading him to believe in a "survival of the fittest" philosophy. Sonic, on the other hand, champions freedom and justice. This creates a fundamental clash between them.
Therefore, while they share a biological connection through genetic manipulation, they're not related in the traditional sense of cousins. They are more like genetic brothers with conflicting backgrounds and ideologies.