What should I do about my sister who's always flirting with my Boyfriend?

I'm a college student and I have a wonderful serious boyfriend who is very handsome and intelligent. He is very passionate and romantic. He's from Germany. I also have an identical twin sister who lives with me and she's taken an interest in my boyfriend too. Every time I invite him over she acts all super nice and close to him, and once she even pretended to be me with him. He obviously thinks she's very beautiful too. I feel rather threatened. What should I do about my twin?