It does sound as if there is quite a bit of jealousy going on probably from both of the other children ,and possibly the oldest instigates the 5 year old to hit out at the 2 year old...as in all families when a new baby comes along and obviously gets a ton of attention the other children feel left out,you could seek counselling as it may help her. I suggest you make sure the older 2 children get plenty of one to one time with you both, that they are getting the same attention they used to get . Take them out by themselves sometimes, make sure their toys and their room is theirs and they dont have to give up toys to younger children thats not sharing ,I have seen older children actually made to give up a treasured toy because a younger child screeches for it.I think the behavior is a cry for help..that she too is still important ,focus on her positive side and praise her when you see her doing something right ..Good Luck.