How do I deal with my negative step-mother?

My father recently married a woman who seems to be permanently stuck in high school. She throws fits like a 5-year-old, she tattles on me, and whenever I tell her something, she runs and tells my dad, but she takes what I said, and re-words it. Yesterday, I told her that I was going to the bank to take out a 6-month loan of $500 to help build my credit, and use the money to pay off my credit card bills. She told my dad that I was selling my car to a title-loan place for $500. He gave me this lecture, and she just stood there with this superior look on her face the whole time. Her son also lives with us. The boy is 21, and refuses to eat with us, so she makes his plate, and carries his food to him! He lives in the outside guesthouse. It's really sad. I'm just sick of her constant lies, her temper tantrums (today she also started yelling about how I'm never going to graduate college, find a job, and she will be supporting me for the rest of her life. I graduate college at the end of 2011. I currently have almost $2,000 saved up, and am currently looking for an apartment.) Anyway, since I'm stuck with her for a few more months, how can I deal with her negativity?