How do I handle a bratty step sister?

My mom just got remarried about 2 months ago and I now have two step sisters. I'm now 16 and the youngest is 7. She is seriously such a brat I can't stand it. She is fat and mean and she stinks. I really just can't stand her. When I have my door shut she just always barges in and I keep shutting the door and she keeps opening it saying that she wants the door open. I have this app on my iPhone kale talking Tom and if you tap the cats head it makes it look like you punched the cat and she starts hysterically crying trying to get us in trouble. I honestly don't know what to do she is a tattle tale and she is always pointing out my flaws like if I have a pimple on my face she will touch and laugh and point it out. I have always been an only child so this is all new to me.