You come over to your girlfriend's house for the weekend, she needs you to watch her pets while she is out of town. Normally her sister would do it, but she is somewhat unreliable and a bit of a mess. So your girlfriend thought you would do a much better job instead. You both are about to head out on a quick ice cream date when your girlfriend's sister spots you and comes to say hi. She asks why you are there when her sister is going to be gone all weekend. A short confrontation about her ability to be responsible ensues and you are awkwardly caught in the middle. There is always a lot of tension between the two of them. You feel bad for your girlfriend's sister though, she is always so nice and friendly. She seems harmless even though she might be a bit.. scatter-brained. Later that night, after your girlfriend is gone for the weekend, you come into the living room to find her sister on the couch. She sweetly greets you and insists the two of you watch something together. You always like spending time around her so you don't see why not. Your girlfriend warned you not to fall for any of her supposed tricks, but what's the worst this sweet, smiling girl could possibly do? She chats with you and complains a bit about how harsh her sister can be, then she excitedly reaches for the remote so that you can pick something for the two of you to watch. Accidentally knocking the remote onto the ground, she bends over the sofa to get it and you get much more of a view than you were expecting. The sight of her panty-less makes you blush really hard. She takes notice and tells you how cute you are, you try to move on and forget about the fact that she isn't wearing panties. She continues excitedly talking your ear off, sitting in various positions that keep you thinking about what she has on, or doesn't have on, under her dress. The conversation starts to take a bit of a turn when she begins asking you questions about you and her sister's relationship. You try to brush it off but you can't help indulging her a bit. Some awkward tension starts to build, but is quickly cut short when she tells you that she will be back in about 20 minutes. Your girlfriend's sister returns wearing lingerie, covered by a sexy little robe. You ask her what she thinks she's doing but she insists that she isn't crossing any boundaries. She proceeds to tease you, taking her robe off a little at a time, and posing in various ways on the couch infront of you. She tells you that teasing isn't cheating, even touching isn't cheating. There are so many ways she could touch you that would never be considered infidelity. She starts rubbing her pussy, telling you that watching eachother masturbate isn't cheating either. Why should the two of you not just enjoy this? It's not like her sister will ever find out about it anyways, and after all, she is the prettier and the nicer sister. You know better but you can't help it, you take your cock out and stroke while she touches herself. She climbs on your lap and starts grinding her pussy up and down your cock, insisting once again that this isn't cheating, it would only be cheating if you actually go inside of her. She takes matters into her own hands and puts your cock in her pussy, but just the tip. That isn't enough and before you have time to consider what's happening she is riding you and you are full-on fucking. She fucks you until she cums. She turns around and fucks you in reverse, begging for you to cum for her, over and over again. You can't help it and you blow your load deep inside of her, she relishes in it and won't let you pull out your cock. It makes you stay hard. She turns to face you and starts fucking you again, while she tells you that she should be the one you're with, and insulting her sister, your girlfriend, repeatedly. You both cum again, somehow you manage another load and let it blow deep inside of your girlfriend's sister. While your cock is still inside of her, she tells you that she is so excited to see what will happen. You snap back to reality and ask her what does she mean by that?? She innocently tells you that she isn't on birth control, so there's a chance you got her pregnant-duh! You are furious, but if you didn't want to get her pregnant deep down, then you wouldn't have risked cumming inside of her. Right??