I still don’t have a name for Ganon being Riju’s older brother. But consider: Riju claims Buliara saved her after Nabooris hit her with lighting. But what if Ganon found her and brought her back. Only to leave again.
Ganon left a few years after Riju was born. He was scared that he would be another consumed by the calamity ganon. His body could be used as a vessel, but Urbosa did her best to keep him safe and raise him to be a good person. He must stay far away from calamity ganon or be taken over. He is not allowed back into the city because of the law. Many people do not realize he exists anymore.