The supposed flash sideways that happened in the finale season of lost was not a flash sideways. A flash sideways is events happening at the same time in two different realms or realities. So on Ouat when Snow and Emma were in the Enchanted Forest and everyone else was in Storybrooke, that was a flash sideways. Nothing that happened on Lost was ever that. There were no other realms or alternate realities, there was no two places with the same but different characters. They were always flash forwards on that show of the same characters.