What do I do when I'm in love with my friend's sister?

I met this girl a month ago on a school trip to vancouver, and although our first couple encounters were awkward, we're now very good friends. But I'm starting to think that I actually love her, which is bad because she's my friend's sister. So now, for the sake of not ruining a good friendship, I'm thinking that maybe it wasn't such a good idea to become friends with that girl. But what's the best thing to do?