Public Nudity/Rip the towel off his waist and see what happens
You hadn't noticed until that moment that the two of you were slowly getting closer and closer to one another. Had you been inching closer to him while you were talking? Or was he inching towards you? Well, obviously something was inching closer to you, you had noticed that! By this time, his bulge was protruding so far outward, that you could probably just stretch your arms forward and touch it. In fact, since you're pretending it's your birthday, you think you just might.
As you start to move forward, time itself seems to slow down. You recall years ago, when he was still living at home, that you had "accidentally" walked in on him while he was naked more than a few times. But every time you had done so, his penis had been flaccid. No matter how hard you had tried, you could never catch him with an erection. You are done waiting for that moment to just magically happen, and so you decide to take matters into your own hands.
You reach down to grab his erection, but at the last second you chicken out and barely graze it. Instead, your fingers clench the top of his towel, just below his belly-button, and yank the towel off of him forcefully and throw it on the ground behind you. And finally, there it is: Your older brother's erect uncircumcised penis stands tall and large, just as you had always imagined. You can't even remember how many times you had dreamed of this moment while pleasuring yourself in your bedroom. You had lost count years ago. You know for a fact that it had been more than a dozen times, because you clearly remember climaxing at least that many times the last night before he had left. You didn't get any sleep that night because you just couldn't stop fingering your tiny snatch! In fact, you remember sneaking into his bedroom naked that night to give him a going away present and you remember how mad you were that he had already fallen asleep. You wanted so bad to wake him up and jump on his dick, but you were only 10 years old at the time and still discovering your own sexual awakening, so you had coward out and instead just watched him sleep for a while as you fucked your tiny preteen pussy with your hands, hoping that your uncontrollable moaning would wake him up. But this time, you had finally proven that you're no longer a coward.
Kevin screamed out "Ahhh! Claire! What the fuck?" as he scrambled to cover his junk with his hands. It wasn't the reaction you were hoping for, but it was still totally worth it for just that glimpse of his peen.
"Sorry", you lie to him. "I just felt weird being the only one here who's completely naked."
"Then go put some clothes on for the love of god! Shit!" He walks around you and bends over to pick his towel back up off the floor. You watch him closely as he presents his ass to you. You can see his testicles as he bends down and grabs the towel. Suddenly, you feel the urge to slap his ass, so without even thinking, you whack his buttcheek with your open palm. It makes a very loud snap as you strike him, and you immediately realize you slapped him way too hard!
"Oww! Goddamn it, Claire! What the fuck is wrong with you?" Just as he's about to wrap the towel back around his waist, you notice the red welt on his flesh where you just struck him.
"Sorry, Kevin. I just missed you while you were gone. That's all."
"I missed you too, sis, but you don't see me slapping that sweet little ass of yours, do ya?" As soon as he says that, you suddenly get a new image in your head. You never realized you were into that kind of thing, but now you're hoping that he hits you back. Maybe he should punish you for being so naughty. No, you wouldn't mind that one bit if he wanted to take your slender, naked body and bend it over his knee and spank your ass repeatedly. Wait--did he say "sweet little ass"? Is it possible that he's as into this as you are?
"Not yet" you reply, then spin back around to show him your ass. You close your eyes, expecting him to strike you just as hard if not harder than you had just hit him. But, after a few seconds, nothing happens. You open your eyes and turn your head back towards him, only to find that he's gone! He had just walked out of the bathroom door without exacting his revenge!
You chase after him down the hallway towards his bedroom. You run quickly because you know he has a lock on his door (although he rarely ever uses it), and you're not about to waste your naked day being locked out of your brother's room. You catch him just as he's about to slam his door shut in your face, but since he's not wearing any clothes to grab onto, you can only pull him back by yanking his towel off him again. Thankfully, the towel prevents him from being able to close his door all the way. You push the door open and drop the towel on the floor as you catch another glimpse of your naked older brother. Only then does it dawn on you that you are finally both completely naked and alone in his bedroom--your childhood fantasy has come true!
"Claire!" he shouts at you. You've never seen him this mad before, and you're quivering at the thought of what he might do to you. So you're going to push his buttons a little bit to find out.
"What?"
"Get out of my room!"
"Why?"
"Why?!? Seriously? Because it's my room! And I'm completely naked! And so are you! And you're my little sister! And you're only 15!"
"So?"
Now he's really pissed. He rushes forward to push you out the door with his big, muscular arms. But since he's still semi-erect, his cock brushes up against your pubic hair, nearly jabbing you in your pussy while he pushes you into the hallway. Then he trips on the towel and nearly falls on you. He is only able to regain his balance by grabbing onto both of your breasts.
After a couple of seconds, you notice that he is sturdy and no longer in danger of falling on the floor of the hallway, and yet his hands remain firmly grasped on your C-cup breasts. They linger there for what feels like an eternity. Just two naked siblings, standing in the hallway, sexual tension building as your brother feels you up. Well, maybe not feeling you up exactly, but he does take an incredibly long time to gradually relieve the pressure off your bosom. For a brief moment, he moves his hands slightly, so that his thumbs rub up against the outer edge of your puffy, still developing nipples. This strikes a nerve ending, sending shivers down your spine and making your vag tingle with excitement. You could tell he was excited too, as his dick began to stiffen again, mere inches away from your pussy. As you look down at it, you can see it throbbing back to life. You think to yourself I could just reach down and cram it in there, but you hesitate to do so. You definitely want your brother's cock inside you, but not like this. However, now that there is no longer any doubt in your mind that he wants to stick it in you, you know it's only a matter of time. Best to wait until he's ready.
Just then, he removes his hands from your teenage titties and takes a step back. Damn! I shouldn't have hesitated, you think. You fear that he's about to walk back into his bedroom and lock you out, but instead he just stands there for a moment, looking at your petite body. After what feels like hours, he finally says "How long did you say Mom & Dad would be gone?"
Your heart starts racing. There's only one reason you can think of that he would ask such a question at a moment like this: He wants to pull you into his room and fuck you with the door wide open! "Until 4 o'clock. Plenty of time..." you look behind him into his room and see his bed, still neat and tidy from the last time you made up his bed while he was gone and you were masturbating in it. And you're convinced that his first time back in his own bed after 4 years is definitely going to mess up the sheets while he rips you apart.
But even though you're a 100% certain he's about to throw you down on his bed and ravish your tight teen twat, he does something completely unexpected: He looks at you and says "Did you say something about pancakes?" Then, before you even know how to respond, he walks down the hall and goes downstairs, still completely naked. You stand in the hallway for a few seconds in a state of shock. Why isn't your pussy being rammed raw right now?
Once you regain your composure, you take a minute to figure out what you're going to do next. The fact that he went downstairs naked would seem to suggest that he wants you to follow him and join him for breakfast. Maybe he's saving you for dessert? You're seriously considering marching down there and begging your older brother to fuck you, you can't believe he'd say no to that. Unless he says no to that. Then you'd be mortified and wonder what about you makes you so un-fuckable to him. You don't think you'd be able to handle that right now.
Or maybe you're coming on too strong? Maybe you should play hard to get. You consider jumping back in the shower and leaving the door wide open, but what if he doesn't take the hint? Or perhaps you'll just wait for him here. It's not like he's going anywhere without his clothes on--you're the one who likes to wander around town naked! Your snatch is ripe and you've just got to get something up there soon. You could always do what you've done so many times before and just start fingering your clit on his bed until he comes back upstairs. What would he do if he found you masturbating in his room? You've always wanted to find out.
So what's it going to be?