Why? Because it's extremely unlikely thatf the specific spearm that fertilized an egg historically would fertilize that same egg decades aftter soemthing major changed in history. The historic Richard Nixon existing in a world where the US lost the Revolutionary War? Swtatistically so unlikely as to be esssentially magic. Generations of Richard Nixon ancestors would have to not only meet each other, and have sex, but they would also all have to have sex at essentially the same time. Worse, all the other factors that went into each of those successful sperm succeeding while all the other sperm that could have fertilized that egg failed, would have to reproduced. That probably means that the sex would have had to be over within seconds of when it happened historically. Good luck with that happening even once, much less three or four times.
So historic figures existing in timelines that diverged long before they were born is a mark of alternate history where the author hasn't really thought through the way things would work in alternate history, or where the author doesn't care.
On the other hand, alternate history lends itself to a uniqu kind of "almost you." The trick here is that while the odds are ubcredibly low that a particular sperm would fertilize the egg it fertilized historically, it;s far more likely that the same egg will get fertilized by a sperm from the same father. Result: somehing almost unknown among humans in nature: Someone with exactly the same genes as the our timeline figure from their mother's side, but different genetics from the father's side. This would be someone genetically closer to their historic equivalent than a fraternal twin, but nowhere near as close as an identical twin.
There are a few rare cases of "semi-identical" twins, where two sperm feritlize the same egg and it splits into two embryos. Usally, the resulting pregnancy doesn't come to term, but there have apparently been a few cases where it has.
Bottom line: If you went into a time-line that diverged a few years or decades after you were born, you could easily meet someone more like you than a brother, but less so than an identical twin. And, by the way, there would be a fifty-fifty chance that the person you met would be of the opposite sex. Very freaky,
Not freaky enough for you? There would also be the possibility of people who have exactly the same generics as you do from the mother's side, but had a different father. The result: someone far closer to you than a half-sibling, and in some ways closer than a brother or sister, but with the interloper DNA.
This could play out in a lot of ways: People with most of your genetics and your name that aren't you. People with your name and your mom's genetics, but a different father (if your mom was fooling around). In extremely rare circumstances you could end up with the sperm that fertilized your mom's egg in your timeline fertilizing another woman's egg in an alternate reality, though given the odds against a particular sperm feritlizing any egg, that wouldn't happen very often.
Hoiw close would you be to your semi-identical twin from another timeline? Would you feel the same ties identical twins have?
Hopefully you can see some of the story potential in all this--unique relationships, infidelity exposed, etc.