I hurt people physically when I'm angry...?

[☺ Cara Lee ☺]

I really need help with this. I know I'm a bad person who is going to burn in hell for being so horrible. I'm a 13 year old girl, and sometimes things just push my buttons so much that I get physical. Only tonight I realized how bad I am. I am easily angered, but I don't physically hurt people unless they give me a reason. Usually it just involves me yelling, or I guess verbally abusing them. My sister, who is 8, is usually the one pushes my buttons to the limit. She steals from me, she lies, and she is all around disgustingly rude to me. I have never actually hit her I think. But she is always the one to start it, she will hit me first, and I act out of anger (which is usually just gripping her tightly, pushing her away, or pushing her against the wall.) I know I shouldn't do that to an 8 year old, and I am setting a bad example which will affect her, but I can't help it. I can't. I really want to stop doing this. I want to stop yelling without thinking. I want to stop hurting people. I don't need you to tell me how horrible I am for doing this, but rather ideas on how to stop doing this. Please... Help me?
I do tell them when she hits me, but they just give her a little slap on the wrist and say don't do it again. I guess I just took things into my hands when I started doing that stuff to her.