What if I'm in love with my best friend's sister?

There is a 2 year difference between the girl and I. I've known her and my best friend for over 8 years. I've made the occasional joke to my buddy that I was trying to **** his sister just to see his reaction but he always says "that's your problem man"...only our mutual friend knows the whole truth, that I don't want to **** her i want to actually date her. I've wanted to seriously ask her out, but I'm too much of a pansy...her last boyfriend was a boring tool, and obviously the family knows me and likes me after 8 years (i practically live there), although I've done some stupid crap at that house over the years. I think her brother, and parents know that I would treat her right. I want to simply ask her if she'd like to hang out with just me, outside of the house...but i don't know what we should do if she says yes....HELP! I NEED AN ADULT!