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?

I think the elder one knows that pulling younger sister's hair is the best defence. I don't think she would have learnt pulling hair from anyone. These things children pick up themselves. In one of the fights she must have pulled the little girl's hair and must have come to know that it is the thing that irritates the little sister the most, and she does it overtime. Also, when there are two kids at home, everybody tells the elder one to behave, compromise, be good than the younger one. The elder one too is a kid. But when there's a kid at home younger to that we always make the elder one let go of toys or things forgetting that 8 years too is young and needs as much importance and attention as the younger one. But yes, parents fighting in front of the kids definitely leave a bad impression on them. If you have an understanding with the parents, advice them how their quarrels are affecting the psyche of the kids.