Responses
They're not blood related.
They grew up together and where treated as siblings since forever. And it's not like he showed up as a kid or a teen and was integrated into the family. No! He was raised since he was a baby by their parents and was an older brother since A was born. There is no way that their relationship isn't creepy and weird no matter how you put it.
What!? Your creepy older brother is being weird??
*shocked Pikachu face*