Help! Boyfriend calls me by my sisters name?!?

I've been dating a guy for a few months now. We hang out with my younger sister, who's 18 and very attractive, and her bf a lot. My bf and I are both 20 but he's mistakenly called me as her name, Kara..which isn't even close to my name, Amanda. It's happened about four or five times already. She is really pretty but enough is enough, right?! He and I are both flirty people but he has tried flirting with her right in front of me..along with this other mutual friend of ours. He's a great guy and we get along perfectly and I really do like him but these few things have been driving me crazy..am I over-reacting?