What would you do?

I ask myself that a lot sometimes when my dad gets drunk and I can hear him trying to get my mom to have sex with him.  She told me she has not done it with him in over a  year now and I'm glad because that reassures me he has not raped her yet.  That's my biggest fear, one time, sometime last year she told me once that if I hear he yelling stop I should barge in and pretend I forgot something or just make something up.  And I did I barged in saying my stomach was hurting, but like when I am sitting here in the living room and I hear my mom telling my dad to stop I always ask myself what would I do if it got out of control ? because this usually happens but he never ends up doing anything to my mom because either she gets up and comes downstairs to sleep or he just gets too tired to argue and try to make her after awhile. Like would I barge in like last time and pretend I was feeling ill or would I barge in and actually tell my dad to his face that I know what he is doing? Would I be brave enough?  I don't know, like sometimes everything is perfect, most of the time we are a normal family, normal financial troubles, normal arguments but than their are times when I feel like we are just messed up and there are things that happen that I would never share not even on here and although they might not be the most horrible things that can or have happened to someone they have always been clear in my memories.  It's times like these when the confusion, hurt and just feeling like I come from a wrong family all flood my head, but like I said we are a normal family most of the time.