Most Helpful Girl
So sorry to hear this. I can only speak for myself. I would confront my dad first. Have your sister present. Be prepared for retaliation and denial. Don't jump into telling her just yet. Seek a professional's advise before approaching her. When she finds out your family life will change for good. She might divorce but it might take some. She might need to pull herself together first.
Regardless of what happens know that you two are in NOWAY responsible for you dad's actions. It's between your parents. Maintain your roles as kids they are still your parents. Stay strong and try to detatch yourself emotionally somehow when the roof caves in.