Your new brother's voice calls back, "Sure, Mom, be down in a bit." You holler out your own "Okay!" While thought for in your head. If you could get a look at your brother's cock, you could get a good idea of what condom to use. Pick a bigger one and get older again, maybe even back to your old self. You step out of your room and over to the door to Mark's room.
Putting your ear to the door you hear noises, almost like a muted slapping noise and heavy breathing. Your younger mind takes a bit to figure out what's going on, but when it finally hits you, you just smile, seeing your chance. Carefully, you open the door just a crack, enough to get a peek at the bed. Your brother is there, hand wrapped around his cock as he pumps away, getting in a quickie before dinner. You take a quick moment to study him, and you have to admit, for a sixteen year old, he's well hung, probably a good seven inches. Letting the door close quietly you sneak back to your room looking at the condoms spread out on your bed. You pick up an orange one that looks like it might fit a cock a bit bigger than Mark's. Tearing it open you pull out the condom, lube once again slicking your fingers, as it does a warming sensation fills your body, your little cock springing to life. You roll on the condom and sigh, the thing loose and dangling from your little cock.
You feel an electric jolt through your body, and the over-riding horny feeling grips your mind. You reach down and grab your cock, squeezing the rubber against your shaft as you begin to stroke it. Soon something begins to push your fingers apart, your cock begins to swell and fill the condom to just the right size, nine inches in length. Your body begins to stretch out, all your bones getting longer and larger, the floor seems to push away from you as you gain height, and your hand seems to grip your cock better as it grows in size as well. Looking in the mirror you see an odd sight, a hairless bean pole stroking away at a large erection, but as you watch the image continues to change. Muscle begins to swell your frame, pecs and a six pack defining your torso as well toned muscle grows down your arms and legs. Your blond pubic hair darkens slightly to a light brown as it spreads out into a fuller bush, which begins to grow in a trail up your abs to your navel. Light brown hairs begin to cover your shins, growing up and thinning out on your thighs, even as more grows on your forearms. More hair grows under your arms as a light dusting spreads out on your chest. A faint, but not full shadow appears on your face which is quickly becoming more adult and defined.
With a grunt you fill the condom with seed and feel your muscles go slightly slack. Rolling off the condom you toss it in the trash and grab a towel to clean off the lube. You take a look around your room, which has changed again. The more childish things have vanished and now sports trophies adorn your shelves, now looking more an older teen's room than a kid's. You look back to the mirror, despite the darkening of your body hair, the hair on your head is still blond, and your eyes still green. You look like the older brother of the kid you just were, or maybe how the kid would look at seventeen or eighteen.
A picture jammed in the mirror frame catches your eye and you take a closer look. Mark is in it, just the way he was when you just checked him out, and two adults your mind recognizes as your parents, and the new you. You stare at it a moment longer and memories float into your head. Mark is now your younger brother, you're a senior in high school now, with a football scholarship to the state college.
Your eyes drift pack over to the pile of condoms... do you still want to go back to your old life, this one doesn't seem to bad. But your thoughts are interrupted by your mother's voice shouting again. "Scott! Mark! Dinner's ready, get down here!" You looks around for some clothes, sure there must be some to fit you.