do they usually leave during daytime or nighttime? do they ever go back to check those babies?
and if they meet each other by accident, do they greet each other, or pretend that they don't know each other?
It's actually a gradual thing done by both mother and kittens starting at around 12-16 weeks, the kittens start to become less and less dependent, the wonder off, stop coming when the mother calls to them ect. It's not done *snap* just like that, it can take several weeks to several months for mother and kittens to grow apart, I've seen it take up to a year before.
sometimes the mother will push home the issue to her kids of "it's time to grow up" and other times it's the mother tailing after her kittens who want nothing to do with her anymore.
as for after the kittens are independent, I've seen it both ways, I've had kittens who have been on their own for a few weeks, suddenly their mother shows back up and they're like "eh, hey whatever" a bit of nose touching, then a not so friendly swat to the face on the mothers part, but other times I've seen kittens completely spazz "OMG MOMMY! ='D" (which is actually kinda cute to watch 6+ month old kittens acting like babies around their mom again) kind of thing, so it depends.
as for do they forget one another, well animals aren't dumb, they know who each other are, and certainly they remember who their mother/brothers/sisters are even when it does come to breeding, they just don't care o3o