My late, beloved Tasha (I lost her a month ago to adrenal cancer) also
exhibited the behavior you describe with Cleo, but to a much more serious
degree.  We got her at 6 weeks.  She was a MF sable and was always very
small (1 pound at her heaviest) and high strung, even as a kit.  When she
was young, the nipping was not too bad but nothing we ever tried ever
really helped.  I finally decided that she was just high strung and snippy,
so we worked to accommodate her temperament.  When she was about a year
old, my two daughters (age 7 and 3 at the time), Tasha, and I were all
sitting in the living room floor playing.  All of a sudden, Tasha lunged at
my oldest and bit into her arm.  The youngest jumped up to get on the couch
and Tasha let go of the oldest and lunged at her, grabbing hold of her leg,
biting her at least twice before I could get her off.  I put her back in
her cage and got everyone some antiseptic and Band-Aids.  Both of the girls
were very upset and didn't understand why she would do that because they
both loved her dearly.  None of us understood what had set her off.  As she
got older, this continued to happen to the children and eventually included
my husband.  They could just be walking through a room and she would
attack, usually bruising or breaking the skin.  I was the only one she
didn't try to chomp on.  It finally got so bad that she was restricted to
her cage.  I would go home at lunch and get her out to play and exercise.
She could also come out if no one else but me was home.  I know this
confinement probably didn't help her mood any, but we never could figure
out why she disliked everyone else so much.  When she was diagnosed with
adrenal tumors, we had her operated on and this helped some with her
aggression.  However, if she was sitting with me on the couch, she would
not tolerate anyone else on the couch and would try to bite the intruder.
To make a long story short (too late?) the best the vet and I could
figure out was that she viewed me as hers and the rest of the family as
competition.  You know, kind of like the little bitty, one person lap
(yap?) dogs that snap and growl when anyone gets too close to them and
"their" person?  Maybe Cleo felt some strong competition for you from your
niece?  Maybe not, it's just a thought and the best explanation we could
come up with concerning Tasha's behavior.  I hope others post about their
experiences with this particular behavior.  Maybe someone else has more
knowledge or insight.
 
Good Luck,
Teresa
[Posted in FML issue 2763]