In Pokémon Omega Ruby and Alpha Sapphire, the player must first receive the Eon Ticket from the deliveryperson in a Pokémon Center. If the player talks to Norman in the Petalburg Gym after obtaining the Mega Bracelet, he escorts the player to the Slateport Harbor. A Sailor will replace the usual guide, taking the player to Southern Island.
So once you get the Mega Bracelet from Steven (when you fight the invisible kekleon in the route below Fortree City), you'll be able to go to Southern Island to catch Latios/Latias.
Hope I helped!