Zym and Ezran

This may be a bit of an obvious question but lm kinda an idiot, in season (i cant remember if it was 5 or 6) why did Zubai tell Zym to: ‘Go back with your brother Ezra’, how is Ezran Zyms’ brother, did Zubai adopt him?! May be an obvious question to some but could you guys please tell me