While the creators of Lost tell us that these "Flash Sideways" are supposed to show us what would have happened if Oceanic flight 815 hadn't crashed, I think it's become quite apparent that this isn't true. There are too many differences between what originally happened and what is happening now. For example: 1. Jack now has a son which we know he didn't have originally. 2. Sawyer is now a cop which would be impossible with the original story line. 3. Miles is also a cop...see #2 4. Ben Linus is a teacher which would be impossible since he was already leader of the Others when the original plane crashed. 5. Alex is a normal high school student (we can only assume Rousseau is the hard working mother she mentions in the episode "Dr. Linus") which, of course, would be impossible since in the original storyline Rousseau was already a crazy lady on the island for 16 years)
I believe the "Flash sideways" are actually what happened as a result of the Swan hatch not being completed. With the changes made by the time-travelling group from Oceanic 6, I think Ben and his dad ended up leaving the island in 1977 during the evacuation led by Dr. Pierre Chang. Perhaps this led to a mending in the rift between Ben and his father.
There are various theories regarding time travel and alternate universes. One that has been shared and portrayed in films and TV is the "ripples in time" theory or the "butterfly wings" part of chaos theory. Namely, that by changing one small detail or a large action, you can change the system entirely. Imagine that a butterfly flaps its wings in one part of the world and the slight change in the air starts a chain reaction that ends with a tornado in another area of the world. I think that the Swan hatch not being built had the same effect in that the people who would have originally died in "The Purge" were instead returned to the rest of the world where they all had various influences in the world.
Just a theory.