Parents are pissed. Help?

My parents are fighting about my stepsister. She is very unappreciative of what my mom(her stepmom) does for her and there's other negative things to say about her but too much to name and my mom is mad. But she's taking her anger out on my dad. I get why my mom is angry but why are my dad and her fighting? I try telling her to calm down but she won't listen. I don't want our family to fall apart. What can I do to calm down?