Sibling Help!! Please?

I am 14 and have a 7 year old sister and a 9 year old brother. My brother always argues with me and get his way most of the time. My sister contently cocks an attitude at me. I told them that if they're not nice to me I'm gonna ignore them. Its hard to do that though. My brother always gets in my face. He'll come up to be and say some weird word loudly. I tell him to stop but he keeps doing. Then I had enough and said stop! But he cried and my step mom was sympathetic for HIM! I had to put up for days with that! My sister says random comments about me. I told her to sop my step mom gave me a hard time. I am really annoyed! What should I do?