Nero and Weiss are brothers, but Genesis is not related to them. Nero and Weiss are brothers but both of them had Genesis' genetic information spliced into their DNA. This process happened to all of the Tsviets except for Shelke and possibly Argento. That's why the Tsviets are reffered to as the 'Hellspawns of G' Because they share similar DNA, they all consider each other brothers.