I see it this way. If he is a good friend to you, then he knows you pretty good. Maybe even better than any of your other friends (or anyone else for that matter). If he is that close to you, then he trusts you, and you trust him. He may have heard you talk about chicks; most guys talk about chicks the way you would expect. Not all guys mean the things they say about women, and even talk about them a certain way when they are with their friends. But, you treat women with respect. Your friend's sister should decide who she wants to be with and that person should earn the respect of her brother (your bud) and that person should respect him. She's gonna decide to be with someone... sometime, somewhere... she will be with someone. If that's to be the case, then who better for her to be with than someone that he already respects, trusts, and knows will treat her the way she deserves to be treated. I know that I would feel that way about my sister. I mean, seriously, you can't help those feelings from happening. Neither can your friend. He can't help the way that he feels about your being with his sister. Just talk to him and tell him what I said above. If he can't get over the way that he feels even after you talk to him about the trust thing, then it's not a good idea, right now, and just lay off the subject for a while. As time passes, one of the two of you will change your mind about the way that you feel. But, at least this way here, if he becomes a true lifetime friend, he will have your respect for doing the right thing. And, if she likes you, your opportunity to be with her won't pass. It will just happen at a better time.