I think I'm in love with my girlfriend's sister (10 points best answer please give your opinion!)?

First off you sound like a really sweet guy and I think you are being sincere, so don't let anybody make you feel shallow. I'm not really sure what ages we are talking about, but if the "girl friend" you have now feels that they need to mature before committing to a serious relationship she may just be nervous to get "intimate with you" This is completely normal for her because if you are the first guy who she has been attracted to in that way then she may be a little hesitant to fully committing to you. This may be why she is hanging around with other guys. She may be trying to feel comfortable with them so that when she is with you it doesn't feel awkward. In addition now that she knows you like her and you know she likes you it might make her feel awkward when talking to you. It's different when you both know your feeling, but haven't expressed them. For the whole sister thing don't be ashamed of talking to her and you should get to know her. It would be best though to establish once more your feelings with the other girl. Be straight forward and ask her if she still has feelings for you and if she doesn't then slowly progress your relationship with her sister, but don't quickly ume that her sister has feelings for you either. If you "girl friend" does say she likes you then confront her about how your feeling about what she is doing. I really hope i helped and things go good for you:)