IMHO the flash sideways are the epilogue.
There is something that our beloved losties still have to do on the island (kill the MiB, resurrect Jacob, become a new Jacob, etc???). When they do this I believe it will basically negate Jacob from ever affecting their lives. What we are watching is the end of all of the characters.
And it makes sense. End of season 5 had Jacob saying "it only ends once. everything else before that is just progress." That is what we are seeing in the epilogue. Progress. Jack is a good father, and putting down his booze habit. Kate is still on the run, but is showing signs of compassion and risking herself by helping Claire. Sayid has Nadia back in his life, and is coming to grips with his past. Locke is finally starting to accept what life has dealt him and getting over his "don't tell me what I can't do" attitude.
And we see that as much as they were all manipulated to come to the island, fate is still pushing them all towards each other. This fits in beautiful with the science vs faith theme. Both science and fate brings our losties together.
Of course, I could be very very wrong in my thought here. This just seems too beautiful and poetic a way to end the show. I don't know where they're going if this isn't their approach.