Emily,
Where did you get Isis?  pet shop?  breeder?  or rescue as a stray?  Isis
is still fairly young, and should not have been in contact with the
newborn kittens.  Young ferrets especially, hear the cries of other
animals, and it's instinctive for them to go after the source.  I even
had the experience of a young female ferret getting agitated when it
heard a visiting friend's human baby cry.  The ferret, normally well
behaved, and didn't nip/bite at all, had to be restrained from going
after the source of the crying.  The ferret was tense and I'm sure would
have bitten the baby.  If there are other kittens still, I'd definitely
separate them from the ferret.  Close the room off at least, or keep the
ferret in its kennel when unsupervised.
 
If you don't have time to work with this ferret, then check with a local
ferret rescue group so they can place it for you.  Most ferrets that
didn't have enough handling and human contact early on, will be bad
nippers, but in this case, perhaps, it's a matter of the kitten's cry
and/or smell that set off the ferret to attack.
 
Phyllis
[Posted in FML issue 4944]