As weird as the situation was, and as much as your girlfriend felt like your little sister now, you still found yourself wanting her.
"Hey" you start quietly. "Even though this feels like incest, does it feel bad?" "What?!" your girlfriend replies, a little shocked. "Well think about it. This is just what we are feeling. None of this is actually true. I mean, we could pretend like all of this is true and have fun, but why stop there. I got to admit that just a minute ago, while we were kissing, I actually liked the feeling. Sure it felt a little weird, but the thought of doing something dirty like this actually turns me on. I don't know, maybe I'm the weird one." you finish, trying not to look at her directly.
You then feel a hand on yours, and look up to see your girlfriend smiling. "Honestly, I don't think your being weird. In fact I only stopped kissing you cause you looked like you were kind of weirded out. I thought you didn't want to go any further because of how everything felt. But yeah, I am kind of turned on also by how kinky all of this is. Besides, when are you ever going to get the chance to try something like this again?" "Seriously?" you start, now the surprised one. "You really don't mind?"
You move to kiss her again, but she stops you. You give her a confused look, and she explains. "We can go through with this, but I want this to be as realistic as possible. From here till the next change you got to call me your sister, and try to think of me as your little sister. When you kiss me, or even look at me, I want you to think that I am your little sister and nothing more, okay?" You nod to her and she smiles. This time you close your eyes and concentrate. You try to think that the only girl in the room is your little sister, whom you loved your whole life. You make up a story in your head quickly about how it was you and your sister who went on this vacation as you don't have a girlfriend. In fact, you start to believe you don't need a girlfriend because you have your little sister.
At this point you open your eyes, and you look at the girl in front of you. This girl really is your little sister you tell yourself. You slowly lean in and gently kiss her. She responds by kissing you back and soon you find yourself embracing her as she strips off your clothing. After about another few minutes of kissing, your sister stops you. "Hehe, now that's what I expect from my big brother. Now things can get really fun."
You watch as your little sister moves slowly towards your hard member, and starts licking and kissing it. You lay back and start to really enjoy yourself as she finally puts it in her mouth and begins to suck. The feeling of your little sister's blow job is amazing, and not before too long you find yourself cumming. You look up to see that she is swallowing every spurt without missing a drop.
Next your little sister climbs on top of you. "That was good big brother, but now it's time to get a little more serious. And remember, mom and dad won't hear us here, no matter how loud we get." And with a smile, she grabs your still hard member and guides it to her shaven pussy. You help your sister by sitting up and grabbing her hips to help aim her. With a little more movement you can feel the head of your dick start to spread her little lips.
In one quick shot, your sister sits down onto your rod and gasps. You had to admit she was tighter than the girl you knew as your girlfriend, but even so she started moving. As her up and down rhythm got faster, you start to move your hips to match her. Soon you could feel a turning in your balls, and you make your final push towards release. Your sister seemed to be giving signs she was ready to orgasm too. You both sit up a little more while still thrusting and bouncing so you can see eye to eye. While in this position you look into her eyes and the only thing you can see in this little girl is nothing more than your little sister whom you love. With this, the two of you kiss as you explode deep into your sister which brings her over the edge as well.
With the act over, and both of you exhausted, you lay back with your sister still holding onto you. You can still feel yourself inside of her, but neither of you care. "Now that's what I wanted from a big brother" your little sister says. "If things keep going this way, I just might want to stay your little sis forever." You smile down at her, and pet her head a little. "Well sis, we are on a vacation right now, but when we leave you might have to become that other person again." With that your sister pouts. "Awe, don't bring her up again. I don't want anyone stealing my big bro. But then again, if that's what my brother wants later, that's what he'll get."
With all that said, you both drift off for a little nap. But right before your eyes shut you look up and the ceiling and think to yourself. "Does fake incest count as real incest if you actually start to believe the person your with IS your little sister?" You look down once more to see your little sister sleeping peacefully on your chest and you realize you don't care, just as long as your in love. And finally you fall into a light sleep.