I've always considered my close friends sisters my sisters. I've know most of my friends since elementary school, so it's kind of hard to really think of them in that sense when I've know them since they were 4 years old, a hard 4, but still.
I just find it borderline incestuous. There are so many factors, but I've always thought it was kind of a scum bag move to go after a good friends sister when you're young. I don't have any sisters so that's just my opinion. The opportunity presented itself a few times for me with friends I made early on in high school(as opposed to my lifelong friends) during a time in my life where I probably would have fucked a sock full of shampoo at the drop of a hat, but was always restrained enough to say no. If I felt that way about a friends sister, I'd probably ask him first before I even pursued it.