Here's why:
* Origins: Sonic is a blue hedgehog from the planet Mobius, created by Dr. Ivo Robotnik. Shadow is a black hedgehog created by Professor Gerald Robotnik, who was working on Project Shadow to cure his granddaughter Maria's illness.
* Different Creators: Even though they are both hedgehogs, they were created by different scientists, and their origins are distinct.
* Separate Stories: While their stories sometimes intertwine, they are distinct characters with their own motivations and paths.
While they are often pitted against each other, their relationship is more of a complex rivalry, sometimes even resembling a begrudging respect. They are not biologically related in any way.