I'm a little confused on when the split took place, I'm thinking 1977 as well,
But I think we have two pre-1977's, one where Jack and crew time traveled and effected things, and one where they did not. Talk about a paradox. I think of it like a two roads that meet in an X shape.
The one timeline without Jack and crew time travel interference results in the plane crash later and them coming to the island.
The timeline with Jack and crew time travel results in a sunken Island.
Or, I'm wrong, and there was just one timeline before 1977 that Jack and crew always time traveled too, and the split happened in 1977.
Actually I don't think about it that much, I just assume a split of some kind took place in 77, and that everyone we know of, including Jacob, Richard, Mr. Friendly, everyone, can or might exist in the flash sideways.
I would argue though, that without Fate and all that, it would be impossible for Aaron to be born like he was in the Flash Sideways, and the odds of the whole conception thing are so infinite against the same sperm and egg meeting up in the sideways universe. So Fate must still be involved in a big way.
Walt wouldn't have been born either after 77, if the side world was pristine and time and chance were random. But the two worlds we know of so far must be connected strongly and time and chance are being controlled by some being or just the universe itself correcting as Faraday's mom said it did when she met up with Desmond on the street bench and the dude with red shoes was under the construction accident.