I wondered about this too. I'd say it's virtually impossible that Flynn would be born after this. It doesn't take much of change to make it so that the one Flynn sperm doesn't fertilize the one Flynn egg, and Flynn is never born. And, this would be a huge change to the specific environment that produced Flynn.
Again, though, there are certain standard conceits of time travel stories. By that logic, you could argue that every time history is changed, it prevents
any of the same people from being born thereafter. And yet fiction routinely shows the same people coming into being even in radically different histories. Marty McFly was still born on June 12, 1968 (and still had the same brother and sister) even though he changed the direction of his parents' lives from 1955 onward. Spock was still born in the Mirror Universe even though Vulcan was a subject world of the Terran Empire rather than a member world of the Federation. And so forth. Realistically, yes, it's more plausible for the grand strokes of history to have a momentum than it is for the same individual combinations of gametes to be guaranteed to occur in variant histories, but fiction does routinely assume the latter, because stories are about characters and so the characters need to be the most constant parts of the story even when everything else changes around them.
One thing I wondered about is how we see that changes in the past don't affect the time travelers. They remember the old history. If Flynn made a change that prevented him being born, would he still exist because he was back in the past when the change was made? At least by the rules of the show? Or, would he vanish on the spot or when he returned to the present?
Usually time travelers are unaltered by the changes in their timelines, and so they can continue to exist after being erased from history. See Spock in "Yesteryear," for example. Or Peter Bishop in season 4 of
Fringe, after a fashion.
And Christopher, we're back to the big/small argument. Yes, having the brother alive likely changes nothing in the big scale. On the more macro scale, though, this is a huge change to his personal family dynamic, and WAY too big a risk to take. He said his mother is emotionally different after this event, moves overseas where she meets his father, etc. Likely makes little to no difference if that sequence doesn't happen, historically, but means EVERYTHING to whether Flynn himself is ever born, no? Maybe the mother only ever wanted a single child, so doesn't have him at all. Or doesn't move overseas, so doesn't meet the father. Or has a child instead of depression, so isn't out bar-hopping to meet the father, etc. History is almost certainly roughly the same, but his PERSONAL history just got a bomb dropped on it. It makes ZERO sense for him to take this risk.
Again, though, it's a standard conceit of fiction that the same people usually do get born regardless of small changes. It's just part of the rules you have to suspend disbelief about along with bigger stuff like, oh, time travel actually being possible in the first place.
And you're offering a lot of "maybes," but it's just as possible to counter those with maybes in the other direction. Maybe Flynn knew for a fact that his mother wanted multiple children. Maybe he knew for a fact that she would've been required by her job to go overseas anyway. After all, he knows his own family history much better than you or I can know his history, so maybe he had reason to be confident that saving his brother wouldn't prevent his own birth. As I've been arguing all along, the problem with singling out a specific factor and insisting it's
required to cause a change is that it exists in the context of countless other factors that might actually cancel out its impact. And as I said, that's the logic that makes the most narrative sense for time travel stories -- that only the
right changes have a major impact, and others have no significant impact.