As weird as the situation was, and as much as your girlfriend felt like your little brother now, you still found yourself wanting her/him.
"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/him again, but she stops you. You give her/him 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 little bro, and try to think of me as your little bro. When you kiss me, or even look at me, I want you to think that I am your beloving little brother and nothing more, okay?" You nod to her and she/he smiles. This time you close your eyes and concentrate. You try to think that the only person in the room is your little brother, whom you always wanted your whole life. You make up a story in your head quickly about how it was you and your brother 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 brother.
At this point you open your eyes, and you look at a boy in front of you. This boy really is your little brother you tell yourself. You slowly lean in and gently kiss him. He responds by kissing you back and soon you find yourself embracing him as he strips off your clothing. After about another few minutes of kissing, your brother stops you. "Hehe, now that's what I expect from my big brother. Now things can get really fun."
You watch as your little bro moves slowly towards your hard member, and starts licking and kissing it. You lay back and start to really enjoy yourself as he finally puts it in his mouth and begins to suck. The feeling of your little brother's blowjob is amazing, and not before too long you find yourself cumming. You look up to see that he is swallowing every spurt without missing a drop.
Next, your brother 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, he grabs your still hard member and guides it to his pink butthole. You help your brother by sitting up and grabbing his hips to help aim him. With a little more movement, you can feel the head of your dick start to spread his little lips.
In one quick shot, your brother sits down onto your rod and gasps. You had to admit, he was tighter than the girl you knew as your girlfriend, but even so he started moving. As he up and down rhythm got faster, you start to move your hips to match him. 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 his eyes and the only thing you can see in this little boy is nothing more than your little brother whom you love. With this, the two of you kiss as you explode deep into your brother which brings him over the edge as well.
With the act over, and both of you exhausted, you lay back with your brother still holding onto you. You can still feel yourself inside of him, but neither of you care. "Now that's what I wanted from a big brother" your little bro says. "If things keep going this way, I just might want to stay your little bro forever." You smile down at him, and pet his head a little. "Well dude, we are on a vacation right now, but when we leave you might have to become that other person again." With that your brother 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 brother?" You look down once more to see your little brother 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.