How do I ask her to the movies?

My girlfriend has a twin. Her twin is around her all of the time, and every time I want to go somewhere, her sister asks to tag along. Of course, for the sake of my girlfriend not getting mad, I say that it is fine. I really want to ask her to the movies, but I want it to be romantic. I want to kiss her. That will be challenging, having her sister sitting next to her or me. It would make it awkward. How do I ask her?