I think I might have misunderstood the boyfriend bit. I thought that the girl had specifically shot James down because she didn't want to be with him, but rather with the guy that she is with (which I suppose still shouldn't necessarily stop you from trying, just be aware that there is a line between "persistent" and "JamesR, you must stay 500 feet away from ______ at all times"). Anyway, if it's a matter of a girl who likes you but happens to be with some dork, then I completely reverse what I wrote earlier: I think you should
go for it. But also keep a few things in mind:
1) It's high school, so it probably won't matter if she dumps that guy.
2) It's high school, so it probably won't matter if she dumps you later. (Not trying to be cruel, just that most people don't marry their high school sweethearts...thank God.)
3) Knowing that you want to date her means that you actually have to go for it. Don't wait around hoping that she'll magically realize that you'd be a great date. You have to make the effort and assume the risk that it'll go/end badly (but refer to points 1 and 2; that's probably not a big deal, all told).
4) If you can't work up the guts to deal with (3) [and you wouldn't be the first to have that problem], then forget about her and date someone else. I like the other poster's "diamond in the rough" observation. There are all kinds of ladies out there, and you never know who you'll really hit it off with until you try.
I'm still not convinced about the friend's sister part, though. I guess it depends on the friendship. You willing to give it up for a chance to date the sister? Potentially get into a really sticky/messy/potentially violent (hey...we all know some families are like that) situation? Then it might be worth it. It wasn't in my experience (not because of the friend, but because of other family members...ugh), but maybe you'll have a better experience.