It can be difficult being an identical female twin, when it comes to fancying boys and having relationships. I think you can rest assured that your boyfriend has made his choice between two cute, blonde twins. While you and your sister look alike, your personalities are likely to be very different and you have to remember that he picked you. He's being extra attentive just now, because he may sense that you feel uneasy about your sister's overt play for him. Her confidence and lack of respect for you, will probably be a turn off for him. My advice is to reduce the number of opportunities for your sister to see him. If he calls to pick you up, make sure you're ready to go out. Don't tell your sister where he's taking you and try going back to his place more often. Plan a holiday or weekend away together and sooner or later, she'll get the message that he's not interested in anyone except you.