Let me break down, break down. We have Domenico/Wes and Enrico Pucci, mixed twins born to the Pucci's. They have different skin tones perhaps due to the parents being biracial themselves. After Wes was kidnapped, his new mom married a black man so the investigator assumes Wes is mixed and therefore black. Of course Wes doesn't know he is dating his sister since she was born after he was kidnapped.
Then yeah, it is surprising the investigator decides to work for the darker twin, but I guess money is money and Enrico is not dating a white girl.