How do I deal with my evil stepmother?

When I was in preschool my father began dating my now stepmother. She was great before she actually married my dad. Then she took a 180 degree turn. I can't exactly what she does, but it's the little things that began to tick me off. Then a couple years ago she had a baby of her own and now I'm just the stepdaughter. I'm 2nd best. Whatever happens it's not about me. I mean they do everything for her. Maybe I'm just being a selfish 16 year old, but I can't remember the last time we did something I wanted to do. Even the simplest things. I mean my dad and stepmom took a week off of work and drove to Pennsylvania to take my half-sister down to Sesame Street Land. I wasn't even invited and it was the talk of the house for weeks. I mean the littlest things like going to the mall, going bowling, those kinds of things. We never do anything I want to do. Everything is about my stepmom and her own daughter. If it doesn't benefit her or her daughter we don't need it. I need help any thing will do!!!