In addition to calling CPS to make a report, you may want to speak to your school counselor or school psychologist about your situation. You will also want to call the Child Abuse Hotline to report what is going on. It is very important that you speak to someone right away so that you and your siblings are protected. Your mother's drinking and the violence that goes on in your house should not happen, and you are right to be frightened for both yourself and your sister. You can also call the crisis center (listed in the front of the phone book) to discuss your situation with someone. Please notify someone right away so that this situation does not continue and you can get help.
Fortunately, CPS is the guardian of these issues. You will need to contact them so that they may begin an investigation. The law requires that CPS and the courts make decisions for the best interests of the children. This will certainly require the assistance of an attorney. You may want to go to the CPS website, as it is very thorough and contains a lot of helpful information regarding your cause. Good luck.