You grew up with Vanessa and because you're about 10 years older than her you've always had a "brother/sister relationship" which was cool, but she did grow up to be a very attractive young lady (and on occasion you thought about "doing her," but never pursued that avenue).
So, you're just "chilling" talking about the old neighborhood and a few current events and Vanessa decides to smoke some weed (you don't indulge) which appeared to make her very mellow and relaxed, perhaps a bit too relaxed because when excused herself to go to the bathroom she stumbled and fell on you while you were sitting on the sofa.
Vanessa fell in your lap and you hands accidentally landed on her breast in an attempt to catch her from hitting the floor. You're not sure if it was the weed or maybe Vanessa was just "horny," but that "brother/sister relationship" was placed on hold that evening.