I don't know your age, but I'm assuming you and your sister live with your parents?
If you are deeply concerned about your parents, I would suggest talking to someone professional and not choosing to deal with it yourself alone. Doing so would only put you in the middle and trust me, having dealt with something similar in my family, it is a horrible horrible place to be. As children in the situation, you don't deserve to be in that position. I don't mean to sound like an old fart, but you're job is to go to school, make friends, work, have fun doing whatever it is you do with your life... not mediate situations like these.
Had I known then what I know now, this is the route I would have taken. Nothing says you can't talk to your dad, nothing says you can't talk to your mom, but you probably know your parents well and what sort of outcome might result. If it's going to cause a big cluster**** of deep hurt, hatred and violence in your family, I wouldn't want to be in the middle of that.