Step-father and mother angry and raging

The other morning my step-father and mother were having a very loud argument. This usually happens about once a week and it is because they are both stubborn and wont give in. My mother eventually gets her way because she rules this house, with an iron fist - what she says and how she says it goes here and he has to do what she wants otherwise she'll rage on him even more. Whenever he does anything contrary to what she wants, she goes off her head at him and that makes him upset and he gets angry and starts raging around the house, yelling and getting angry at me.

My mother has this insatiable need to control everyone and everything in her vicinity and if they don't obey, then she has something to say about it. Most people don't get to see this side of her, they only see the pleasant, amiable persona she decides to show them, but if she feels thwarted, they get to see this antagonistic, bullish self and they do not like it at all. There are quite a few people who give me the "you poor thing, she is your mother and eye-roll" when they realise who my mother is, because they have been subject to this side of her personality and they really don't like her, but because they don't know me, they don't say too much more than that.

Anyway, during their argument, my step-father started banging doors really loudly, started yelling really loudly and started swearing really loudly. Of course because he was upset, he didn't think about the fact that there are sick people who are still sleeping (me) and he closed the toilet door which is just off my bedroom really loudly and he had the fan on too. He doesn't really care about anyone else, as he is rather very selfish.

To top it off, my mother wont tell him to stop doing what he's doing, out of consideration for me sleeping and being sick needing the sleep, she'll upset and antagonise him more, telling him he's a no-good-for-nothing, lazy, layabout and she doesn't know why she married him and more words like that. He on the other hand starts telling her how lazy and good-for-nothing her children are and then she goes one to tell him that he knew she had children when he met her (we were young when they met, about 9 and 11 years old) and that if he didn't want to be here to just leave because she certainly doesn't need him in her life.

Needless to say, these arguments are not very nice and many times I have tried to stop them arguing by being impartial (not taking anyone's side), trying to get them to see each other's side and trying to get them to see reason. Usually it has worked as I know how to use the right words to diffuse the situation, but these days, I really cant be bothered - it is not my problem to solve, I am not responsible for their marriage resolutions.

When they have these arguments and either of them sees me - my step-father, because he is angry, will just say to me: "You lazy bludger, good-for-nothing, go back to bed and sleep" and my mother will just look at me with an angry face and if I say anything to her, she will just tell me off. It's better not to even approach either of them when they are in this type of mood.

Inevitably I am always blamed for their arguments - they both tell me that I am the cause of their argument. So again, neither of them take responsibility for their relationship and want to blame me, because I am there. Even when I am not living here, they still blame me!  Geez, when will these people learn how to behave like decent, normal human beings?