I totally see what you're coming from. I think you should trust him because I was on the other side of that. My best friend (who is a boy) has had a ton of girlfriends and they have all gotten really used to the fact that we hang out all the time. If your guy is saying she is like a sister...trust him. If his best friend is being mean to you talk to him about it, or her!
I am in the same position now only a little different. My partner and our friend (woman) have all known each other for 26 yrs and I have recently hooked up with my partner 2 mths ago. Yesterday my partner told me that our friend had spoken to him yesterday and had said "just because you are seeing each other dosen't mean you can't come over and drink with me!! Knowing he will not drink drive and would have to stay over the night.. Now I don't know if I'm comfortable with this as I know they have slept together before as they had both told me which is fine!! But this woman has also in the past slept with my daughters father and has a really bad reputation with men.. She also went funny when we got together and refused to talk to me saying 'you got your man now, you don't need me' but has since apologized and said she was just in a mood!! I feel that because I have been saying what a lovely man he is and he was special she now feels like she should have him.. She is 10 yrs younger than I and is very attractive!! I spoke to her yesterday and said I should come over for a drink one night but she didn't seem to keen and I feel that as she knows we are together the offer should have been for both of us to go not just my parter,, please let me know what you think!!!
i have the same situation....i always see her as a friend..like a sister...she also sees me like a brother.i would not give away my love(if i had)for her........but in the same way i cant leave her..and i am sure u r boyfriend shares everything with her from the start......he would like her just as a friend but..i bet he wouldn't like to marry her....see a small line is present between love and friendship.u should understand that.....