What should I do about my Sister 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 has taken an interest in my boyfriend too. Every time I invite him over she acts all super nice and close to him and I've caught her flirting with him several times. He obviously thinks she's very beautiful too. Should I feel threatened and what should I do about my twin?