I hate my step mom at times?

Ok, well I'm 13. My Dad remarried about 6 years ago, and she used to be so nice and relaxed. Now everyday usually she's in a bad mood after work, which makes everyone else mad... My stepmom has a daughter and son that are her own, and then there's me, and my two brothers. I guess it really aggrivates me that she will do anyhong for her own daughter, and she's known me since I was a little girl. Todaymy brothers were arguing, (they're 11 and 14, pretty girly though) and the younger one started kicking the older one, and he was just sitting there! I decided to try and prevent. Black eyes (happened before) and break up their little cat fight. Then my step mom walks in, pissed as usual. She then made a comment about how my older brother and I need to grow up!! But the kid who attacked him doesn't need to grow up? Wow. I feel like jut because we arent her kids, she feels like she can do anyhing and say anything to us. Don't get me wrong, sometimes we do get along... Just not all the time. Maybe it's just me being sassy? (which is what she called me) how should I handle this? It just makes my life harder.