I've read your other posts, Sarah, and i have to tell you - step families are extremely difficult to adjust to at the best of times. Even when everyone gets along, there can be an undercurrent of tension or small incidents where someone says something hurtful or disrespectful. So in your case, where the new girlfriend is close to you in each, wow - that must be very hard to deal with and just as much for her as for you.
If your dad is really serious about this woman, he needs to be the one to make sure everyone he loves is on the same page. That may take a while. He can't expect you to immediately defer to her, and he can't expect her to adjust to suddenly being a "mother" of two teens when she is barely out of teenhood herself. I think all of you should have a meeting about this and put some honest feelings down on the table. All your emotions and hers are completely valid. This is not a small thing.
Counselling is not always easy to get but it may be the only option. You do need some third party to sit and listen to you all and sometimes just talking about it helps. You all need to be honest though, without being disrespectful.
As for leaving, that's a tough one. You have to think about financial independence and future plans. Before you decide to investigate that, why not give this woman a chance? She is probably feeling as lost as you are.
What I'm trying to say, is don't rush into it. Yes, leave home if you're unhappy, you're certainly old enough so it's your own decision. Just make sure you're ready both mentally and financially. And make sure you have a plan.. don't just think you'll live in your friend's spare room because that never works long-term.
your eighteen.. walk right out that door. but be prepared for "real world" when you do.
Thank you for the advice, I appreciate it
Thanks so much for your advice