A good firm know and ignoring the cat when she does things like this
may be helpful.  You really need to get the point across the children
are higher on the food chain than she is.

I don't think having your sister spayed now will help.  Had this been
done a while ago, there would be no children to attack and no problem.
Too late now.

So, now what?  Having the kitten spayed is a good idea regardless:
shelters are already overflowing with homeless cats.

Kittens are playful.  They grow up much too fast and the playful days
dwindle.  Enjoy them while they last.  Cats are very smart but their
grasp of human language is limited (or so they let on).  They really
don't want to do things that might get them orphaned at any age.  They
respond to kindness and patience.

Two kittens are five times the fun of just one.  And they have each
other to attack, which saves wear-and-tear on the human kids.

Roby

Yes, she needs to be spayed, but that won't stop the kitten's need to
play. Are the kids playing with their hands with the kitten? They
should be using interactive toys, so the kitten learns that biting the
toys is ok, but hands are not. It's ok to say "no" or a loud "ow" when
things get too rough.