My niece scratches, pushes, kicks and pulls her younger sister's hair everytime they fight and would say things an 8yr old shouldn't say. It is all because of what they heard and saw whenever their parents quarrel. I don't want them to grow up like that. Is this just a phase or should I be worried? Should I consult the kids to an expert?

Kids tend to mimic whatever they see and hear as it is natural for them since they're still in the process of learning. What they see -- they do. For this, I think the best course of action is to talk to their parents. My eldest (born from a different mom) grew up with his dad (my current partner) and his wife (ex) always fighting. When I first met him, he was always craving for attention. He also says words that a (then) 7-year old boy shouldn't be saying. It took me a while but when I finally got close to him, I learned that he was that way because he thought it was okay since he sees his parents talk that way. I guess what I'm trying to say here is that, you can do something about your nieces by talking to them but eventually, they'll just do it again because of their environment. It is really best to get to the root cause first by talking to their parents and educating them about the effects that is happening to their kids.