Off limits or fair game

90% positive that's off limits. Unless your best friend hates her. Then proceed with caution,

Yeah thats what i was thinking too.

Nice username btw

Thanks man,and good luck with your best friend's sister!

It's off limits depending on age. If you're at a time to get really serious and consider going the distance, it's fine. Otherwise there's a fair chance one of you will break the others heart, putting said friend in a bad place. Typically siblings are off limits. Twins or not. But you can still like her! Best of luck

Yeahhh kind of figured. I mean i would like to do that of course but I'm not sure how realistic it is considering my age.

I say go for it :)
You'll only know if you try! As much as I admire a Stinson style rule book, I say fuck it (here no pun intended) and do what you want.
Talk it over with your friend, then talk to the sister :P, there you go, enjoy my worthless wisdom :)