I'm not able to accept I am having a step sibling and strongly dislike my stepmom
Can you explain your family dynamic to me? How does your girlfriend fit into this picture?
I guess I'm just scared that she will treat me like her sister and not as a partner
I know step families can be quite daunting and challenging, but you have to do your best to have a great relationship with everyone. You don't need to be that majority of unhappiness and drama.
I will do things my own way
It can be rough to have a blended family sometimes. I feel that if you talk and are open with everyone things can run more smoothly. If it gets bad you can just remind yourself soon you will be an adult and can move out.