You see her from time to time when you go over to your girlfriends house, but you don't really know much about her. She's always been a bit mysterious, which intrigues you. One day you decide to stop by unannounced to surprise your girlfriend, but she's not in her room, so you go looking for her. Maybe her sister would know where she went. You pop your head into her room and glance around, taken aback by all the games and posters (she must be a bit of a nerd or something). She's working at her computer on a site called Patreon, uploading what appear to be lewd photos of her with a tail or something. You shake your head and tap her on the shoulder, asking her if she knows where her sister is. She has no idea, and for a moment your eyes briefly meet. She glances away and goes back to her uploading, but you know she was thinking the same thing when your eyes met. "You know you've wanted this..." you hear yourself say distantly, and that's all it takes. Before you know it she's on you, riding your cock harder than anyone has in your entire life. You could never get her sister to get on top but you didn't even have to ask. You didn't bring a condom and you will yourself not to do it, but before you know it you're cumming in your girlfriend's sister.