Okay now I have witnessed the biting my sister was talking about.

Princess holds phoenix in her paws or arms and starts grooming her, then the hold gets tighter and she starts biting her head or neck (that's usually what she is grooming at the time) while her back legs are kicking at Phoenix.

Cats usually do this when you play with them. When they are on their backs and you play with their stomachs they start to attack your hand while kicking. This is what Princess just did but with Phoenix.

I'm not sure if I mentioned before but Princess has been extremely playful since she gave birth. Could it just be her playing?

My sister also said sometimes she will watch the box with her tail swishing then she pounces on Phoenix and Phoenix will usuall meow and climb out from under her. Princess doesn't attempt to attack her along with the pounce, its just a pounce.