Why does my brother let his girlfriend boss me and him around?

Well I'm 14 and my brother is 26 and his girlfriend is 29. I live with my brother and his girlfriend because my parents are deceased and my brother's girlfriend is pregnant at the moment and that is making her even more of a b*tch than she usually is. Anyway my brother's girlfriend decides what films we see when we go to the cinema and she also decides what music I listen to, what games I play and what films I watch. She also won't allow me to watch 18+ films or play 18+ games so I have to do that behind her back. Anyway yesterday I was playing Grand Theft Auto IV and I was on a shooting spree and I shot many people. But my brother's girlfriend comes in my room and takes the disc out of the Playstation 3 and she said "sorry hun you can't play that game because your only 14". I then said "f*ck you I'll play whatever game I want and I'm tired of you bossing me and my brother around". She then slapped me across the face and grounded me for two months and when I told me brother he said "whatever she says goes". Why is he letting her boss me and him around?