keep her in the friend zone, if she likes you and you break all contact she will come after you and stalk you for a while if you can deal with that its fine, but a teenage girl following you around and e stalking you isnt the best of looks now is it, for example she could change her status on face book to in a relationship and say shes in a relationship with you and well rumors ahoy!, if shes not then there isn't a problem, she more than likely wont try anything judging by the post you have made before she doesnt know how, and if she does try something be nice and tell her that you dont feel the same way, for all you know you could be her first crush, and well you dont want that to be a awful memory that will scar her for life now, would you?.
if youve already got a girlfriend an easy way to do this would be just to take her along some time and feign ignorance that his sister likes you, proceed to act all lovey dovey (i cant beleive i just posted that word there) with your girl and well (not to the point of making out of course you have to act subtle so she doesn't think that shes absolutely destroyed here, just hold hands or something) , his sister will get the picture and give up hopefully, and if she doesn't the rumors will be abated with the hole youve already got a girlfriend thing.