Oh, so your best friend is a girl with a hottie sister, eh?! And you once had a thing with the girl who is now just a friend. So the "friend" doesn't know that you're scoping her sis, but does the hottie sister know you have a thing for her?
Yeah, I can't see this ending well no matter what. But tell us which one is cuter and just with that, more than a couple people here should be able to tell you pretty much how the drama is going to play out.
About the closest thing I have to a best friend now is also a girl (woman?). But we've known each other for about 15 years now. She's very attractive, but it's always been platonic between us. And she's the one who introduced me to the girl I've been seeing for over a year. The friend doesn't have a sister, but if she did and I hit on her... I think she'd deal with it. But I don't have the best history with women, so I know I'd lose a friend somewhere along the line. I mean, "blood is thicker than water" and all that. So at the (inevitable) break-up, I would expect her to take her sister's side.