Meliodas is the eldest. They both hate the Goddess clan because they've slaughtered their people. Zeldris wants to become demon king so he can make everyone in the demon realm equals
 Zeldris is the Commandment of Piety. Zeldris deeply respects Meliodas 
Meliodas is the Commandment of Love
Meliodas will do whatever the Demon King commands. He cannot feel emotions. He's the leader of the ten commandments. Zeldris is more shy than Meliodas
They are brothers and both are sons of the Demon king