So this might make you tut, but what are the unspoken 'rules' around sleeping with your friends sister?

Over the last 6 months I have occasionally spent time with one of my friends younger sister, usually on nights out or whatever, and the flirting/levels of interest seem to increase every time. She's only 19 and is a lot of fun to socialise with because she's pretty wild, definitely not what I would consider girlfriend material, but she does seem to have a sweet side to her. If she wasn't related to my friend I would probably pursue a fling, but I guess this would be deeply inappropriate and disrespectful to him. I have known him for 15 years and he is a dear friend to me, and I am also on good terms with his family who were like a surrogate family to me when I was growing up. So do you think it would be a fair assessment to say the only way it would be acceptable is if I wanted a serious relationship with this girl (which I don't), and it's the forbidden fruit factor that is making me think about her in this way?