Your younger sister, Isabella.

You don't even have time to react before your little sister, Isabella, runs into your room and jumps on top of you. She lands on top of you with a big smile on her face.

"Happy Birthday, big brother!" she yells, as her hands start moving across your ribcage - her fingers moving like little spider legs as she starts tickling you. You start to laugh uncontrollably as you frantically flail about, trying to keep her quick, little hands from finding your most ticklish spots. Realizing your only hope is to fight back, you reach up and start tickling her sides, and she too begins to laugh uncontrollably.

"Ahhh! Zack, stop!" she yells in between her laughter and gasps for air. As you continue to fight back, you suddenly realize just how cute your little sister is. Her slightly wavy blonde hair hanging down above you, her brilliant blue eyes that seem to be able to stare into the deepest parts of your soul, and her adorable smile almost put you into some sort of trance. It doesn't last more than a second, however, as you're suddenly snapped back into reality when her hands find their way into your armpits, your most ticklish spot, due to your lowered defense.

"AAAAHHHHH!!" you cry out while laughing hysterically as your body starts involuntarily convulsing, trying to do whatever it can to escape your sisters onslaught of tickles. You quickly roll to your side, throwing your sister onto the bed beside you. You try to maneuver yourself above her, but with a surprising amount of strength, she uses your momentum against you as you over shoot your goal and roll over, and past her.

Your eyes go wide as you realize that you're not going to land on the soft surface of the bed. You've reached the end of the mattress, and in a panic, you grab onto your sisters arms as you're flung over the side of the bed, pulling her down with you.

THUD. The wind is knocked out of you as you, and then your sister on top of you, slam onto the hard floor. You stare wide-eyed at each other as you get your breath back before you both suddenly burst into laughter. Your sister props herself up on her elbows, still lying on top of you, and looks at you with a cute smile on her face.

As you look into her beautiful eyes, you can't help but notice how her body feels pressed against yours. Only standing at 5 feet 4 inches, your sister has a pretty small build. What's not small however, is your little sisters bust, which is now pressed tightly against your bare chest. You're not sure what size they are exactly, but if you had to guess you'd place her bra size around a 32D. Speaking of which, you suddenly realize she must not be wearing one as you feel two bumps, each about the size of a pencil eraser, poking against you.

You feel your face getting red as blood flows into your cheeks, hoping your sister doesn't notice as blood also begins to flow into a more intimate part of your body. You could have sworn you saw brief expressions of shock and then shyness sweep over your sisters face as your member quickly reaches a semi-hard state of arousal, but it's gone so fast that if you had blinked you would have missed it. Maybe you're just making it up in your head, you think to yourself, because she's acting completely normal.

"So, what do you want to do for your birthday, bro?" she asks as she bends her legs at the knees, adorably kicking her feet back in forth in the air.

"Not getting crushed by my sister would be a good start." you reply with a smile on your face while trying to think about dying kittens, Steve Buscemi, or anything else that might keep your swelling member at bay.

"Hmm, but crushing you is so much fun!"

You grunt as she bounces up and down on you, trying to ignore her lower stomach rubbing against your crotch with every movement. You're starting to lose your battle with your body, and you know your boxers aren't going to do much of anything in terms of concealing an erection. "Good god, I should not be having these thoughts or bodily reactions towards my own sister." you think as you quickly try to come up with a way to get her off of you and out of your room.

"Isabella, Zachary, time for breakfast!" you here your mother yell up the stairs.

"Yes! Saved by the proverbial bell!"

"Looks like you lucked out this time," Isabella says with a smirk as she starts to get off of you. As she does, she straddles your leg, and for that brief moment you swear you feel a damp, incredibly warm heat against your leg. Which, of course, sends a shiver down your spine and a sudden rush of blood into your quickly hardening manhood.

Your sister stands up and looks down at you for a moment before spinning around, giggling, and exiting your room in a movement that you can only describe as prancing. As she leaves, you can't help but stare, and see she was only wearing a pair of black, skin tight boyshorts and a dark blue shirt with white polka dots on it. The sight of her tight butt bouncing with every hop as she left your room quickly gets you to full mast.

Not wanting anyone to see you in this condition, you hop up off of the floor and shut your door, making sure to lock it just in case.

"Better get dressed and head downstairs for breakfast before mom gets upset."