Your niece is a projection of some part of yourself. Look at the negative traits you see in her, and ask yourself "What of these are also things I see in myself?"

Something amid these shared traits is what you're relating to the destruction your niece is putting out. Anger in dreams is often a more mild version of a communication problem (something that violence is a more direct action against). "Is there something I'm dealing with where I'm at odds with someone or something?"

That the anger is suppressed in the dream is nice, but it doesn't necessarily mean the issue is over. Addressing the root of the anger/oppression/aggression is important or things will only bubble back to the surface.