Problem with daughter

My daughter has split my family in two, she seems to be pulling my ex strings and is giving us all a life of hell, i have had to move into my mothers with my son and my two daughters are living with my ex, she is making my youngest daughters life hell and she wanting to get away from her and live with her friends, how do i get to tell my ex that it is my daughter who is splitting the family up and she doesn't care.