Attraction to a friend's sister

Well this is my first post, hopefully I can get some good advice. I'm just caught between a rock and a hard place. I've been attracted to my best friends sister for a few years now. When it first began the feelings were mutual between she and I. My buddy wanted to kick my ass when I tried to tell him how I felt. I immediately cut things off between she and I for the sake of my friendship. I still have the same feelings now, a couple years down the road and really would like to pursue something with her (his sister). Any advice on how I can make it work? Convince him that it will be okay, and that I'll treat her well. I can't help how I feel. I'm really lost on what to do. Thanks for any replies in advance. :)

Nice one... I tried to tell my friend that his sister is my gf(even no) to see his reaction... Of course, he will say no, but if she became your gf...Your friend maybe just say "ok",hahaha