I don't think that this is aggression at all. It sounds like she is just being a puppy, and at her age Bella is coming into the teen stage and is simply pushing her boundaries and acting up to gain your daughter's attention. She is testing to see what she can get away with and you need to show her that it is unacceptable before she pushes it any farther.
Now it seems as though your daughter is the primary caretaker of the dog in your eyes. But your daughter being only 12 yrs old herself I don't see how this is really possible to raise a well-rounded dog when you are the one home with the dog most of the day. I think that you need to step up and help your daughter with this since she is still very young and IMO should not be fully responsible for this dog quite yet. Puppies take a lot of work, especially at this age where the pup is getting older and will be pushing her limits.
There are several other threads concerning this same type of behavior with young boxers; here are a few very recent ones. Maybe you'd like to read over the advice that others gave to these posters, and then sit down with your daughter to talk about how she should handle Bella and what you are going to do to help her.
Should I worry about this behavior?
what do I do?
- Fawn Female, born 03/23/2001
- White Female, born 03/27/2007