I second that, I've come to be more or less my little sisters father figure. I sit her dates down and give them a hard enough time and scare them how it is I would probably go crazy if one of my friends dated or umm tried to date her.
Either way I think dating friend'* sisters is a bad idea, I can only see that as a conflict of intrest.
Well, as a couple can attest to here, I treat the girls with a lot of respect. I don't often consider dating a friend'* sister, but have twice in the past. One ended good, and one ended not so good. The not so good one was strained during the relationship, so that wasn't really in my control.
I also don't have a sister, so I am not on the other end of things. But I figured since I am 1 for 1, I might as well ask