Dog abusing niece?!?

If she kicked your dog right in front of you and everyone, then she doesn't need to be sneaky. She can obviously get away with anything. I'd personally take her aside when your mother and sister weren't there, and tell her if she harms your dog, you will break her snotty little neck. I protect my dogs from ANYone who would harm them, relative, stranger, or what-have-you. It is your responsibility when you take on a pet to protect it. It's a wonder she didn't kill that poor pom. If stopping your niece means her mother has to find another sap to babysit her heathen, so be it.