It will probably stop on its own.  The kittens were weaned a bit too soon,
which is contributing to the suckling.
If you are concerned, you could get the girl kitten some type of lovey
stuffed animal and when she starts suckling, transfer her to that.

-Kelly

Hi, Ansi...

Just wanted to chime in on this subject as I've always done so often in
the past. You must put a stop to this queer sick behavior immediately.
Scold her if necessary. Keep hard at it. Don't let up. We've got a full
grown 'baby' here now that still thinks that the wife's neck-mole is a
ninny. It's because I let it go and go because she thought it was
'cute.' It is sick. It is akin to a child who constantly masturbates
and a mother who knows it but lets it go on. A security blanket of
sorts. And if we want kitty to be healthy both in mind and body, we
will stop it from happening immediately!
Don't listen to the warped ideals of those bleeding hearts in this
group who will try and tell you that it's okay. They tell their own
kids that whatever they 'choose' do is okay as well. To them, there IS
no 'right or wrong.' All is good, since there is no bad. No rules...
Just right. They are ALL warped obsolete props left over from the nutty
1960s, so don't listen to them. Listen to ME. I'll never steer you
wrong.

                       IBen Getiner

Unfortunately, the kittens were adopted out much too young.

They really should stay with their mother until the 12 to 16 week period.

The behavior is normal and will likely diminish with time.

My two kittens were orphaned at 3 weeks and I got them at 6 weeks with the
same problem.  I got them KMR and a bottle and they had 2 bottles a day for
another 7 weeks, and then spontaneously weaned themselves.  How they
clamored for that bottle!  I have some adorable pictures of feeding them.
And it was sweet to have them sleeping on my shoulders while I watched TV at
night after feeding them.

They are now 15 and two of the sweetest cats I've ever had.

That behavior is not a problem, it's just a behavior.

Our kittens are 13 months old now, and still try to nurse from their mom
now and then. There hasn't been anything there for many months, but they
still try, and purr up a storm in the process! The kitten we adopted out
at 4 months old tried to nurse from his companion cat when he was a year
old. The kittens we kept, also once in awhile root around in their
brothers' or sisters' tummies, hoping for a snack.

I think it's just fond memories, they definitely aren't underfed!

Rhonda