In this predicament myself, we aint that close but he's a cool lil nikka. His sister got ass out this world and I know she's feelin me. I only hit her up to cop bottles for me, but I know if I ever end up gettin faded around her its goin down 

I'm also in a similar situation my friends sister is throwing out those signals and is really cool but I won't do it theres to many chicks out there why even possibly put a strain on your friendship.
Don't do it. I'd never date one of my brother's friends. There are too many guys out there for me to ruin my brother's friendship.
edit:Mantle I didn't even read your comment before posting.
Don't do it breh. I had a boy we were real cool in H.S. Not as cool now be we're still friends. Anyways his sister used to always be on me, even when she had a man. She used to tell me that she would leave her man for me blah blah blah, but I would resist cause thats my mans sister. One time I was at dudes house and somehow I was alone with her in her room and she threw me on the bed like she wanted to start something and again I resisted. Finally when I was about to go away to University she wanted to come over to my house to say goodbye. we ended up chillin and watching tv then I coudn't resisted no more....so we started kissing and rubbin....it led to my bedroom. I was rubbin her coochie and stuff and when I was about to hit she was like "only if you call me your girl and I call you my man can you get it". I was like, are you crazy!?? Anyway to make a long story short I went away to school and my boy and I haven't been as tight since. In fact dude never came to check me or anything while I was away. So I dunno if he found out but it seems like he did and it ruined our friendship.
BTW here's the girl:
guavaleaf.com - guavaleaf 'I Got...' The Ad. Campaign (Behind the scenes)
Monique Crossfield - Actors' Reel on Vimeo