Hello I am sorry O is mean to you and your family. You do not deserve to be emotionally abused by a bully. Your mother is responsible for O being there in the home, correct? Domestic Violence Crisis is a repeating cycle and many women do not have the strength or skills to properly protect their children (or themselves, at first). The support of a DV crisis supporter would give your mother some options and help her form a safety plan for herself and her sweet children (you and your sister). It is natural for you to want to protect your family from him. Get local help from someone in your area and hook up through your T (or get assigned to one) for coping skills. Abuse is always the fault of the abuser and their codependant NOT the children.