Yes, now is the best time for her all around, but it is considered young by
"industry standard"  Find someone that does pediatric spaying.
http://cats.about.com/cs/spayneuter/a/earlyneuter.htm

the biting is natural play for the kittens.  They were taken away from their
mother a little young, and were not taught that there is a limit to biting.
Recommend age for removal from the mother is 12 weeks.  some breeders refuse
to let kittens go before 16.  Since she is already gone, your sister will
have to become the new parent and teach her when to reign in.  first step
would be to meow in an OW like fashion.  This should cause the kitten to
stop what she is doing and look quizzically at you.  She may go right back
to biting, do it again.  If she doesn't stop, put her down and give her
absolutely no attention for a few minutes. Do not scold her for biting, she
wont understand and it is natural for her to do so.  If she comes right back
while you are attempting the no attention part of this, pin her gently to
the ground using the scruff area of her back. (right above the shoulder
blades) scruff slightly if she is squirming.  Scruff for only a moment or
two, then go back to the no attention for a few minutes.

The meOW should do it.  Ive had persistent biters before.  You have to teach
them that nibble is ok, that bite is right out.  if you have gone through
all of the steps repeatedly for a few weeks, then you can start with the
short scolding, and very very light taps on the nose.  You'll read that its
not ok to hit cats, that you ruin their trust of you.  I however have found
that if cats don't have the "fear of god" of you, you can't keep them from
doing things that would most likely end up with their getting hurt (like
jumping on the stove)

Please wait 6 months. That's what my vet, and those of my friends, have
always recommended.  It won't hurt her to wait until she's 6 months.

Beth