It's funny… one of the kittens is still nursing at every opportunity (like a certain almost-3-year-old I know), and I think the other one is completely done. Even in the feline world, some people are ready to give it up sooner than others.
Interestingly, the kitten who is still nursing is more outgoing, more adventurous, and more curious than the other. The earlier weaner seems to be generally self-contained and content. She's more tolerant of M.M. mauling her and carrying her around, and has never tried to go outside.
So apparently (to my mind, anyway) the continued nursing gives that kitten support that allows her to be more daring and independent, so long as she can come back to Mom and get what she needs periodically. Though it looks to me like Mom is getting a little sick of it, and usually gets up and walks away after a very short period of nursing.