don't mess with friends' sisters. most brothers are pretty proctective, and even if she liked you a lot, she'd probably know better than to go out with you, especially if she knew her brother would kick your ass if she did. and you should be thankful for that because it would probably be for your own good. not trying to be cynical here, but in my opinion, it's off limits.
Can't fight darkness with darkness, only with light.