You see what happens is that you have a child and she is the apple of your eye she gets all your attention, then a few years later a little sibling appears, gets all the attention babies get,quite naturally, your first born feels hurt, usurped, this is the time to make her feel as loved as she has always been and I am sure parents do what they can, and sometimes its a normal progression ...She is fighting with him because she feels some jealousy, I suggest you have one to one time with her when he is doing something else, get Dad involved with playing, games , outside activities , Fun , reading board games and walk in her shoes .Good Luck